Add the errors key first in the response JSON

According to the `Response Format`
[section](https://facebook.github.io/graphql/June2018/#sec-Response-Format)
of the GraphQL Spec:

> When errors is present in the response, it may be helpful for it to
appear first when serialized to make it more clear when errors are
present in a response during debugging.

refs andreas#119

graphql/src/graphql_schema.ml

@@ -1148,12 +1148,16 @@ end
     in
     (`Assoc (("message", `String msg)::props) : Yojson.Basic.json)
 
-  let error_response ?path msg =
-    `Assoc [
-      "errors", `List [
-        error_to_json ?path msg
-      ]
+  let error_response ?data ?path msg =
+    let errors = "errors", `List [
+      error_to_json ?path msg
     ]
+    in
+    let data = match data with
+    | None -> []
+    | Some data -> ["data", data]
+    in
+    `Assoc (errors :: data)
 
   let rec present : type ctx src. ctx execution_context -> src -> Graphql_parser.field -> (ctx, src) typ -> path -> (Yojson.Basic.json * error list, [> resolve_error]) result Io.t =
     fun ctx src query_field typ path ->
@@ -1234,8 +1238,8 @@ end
     | data, errors ->
         let errors = List.map (fun (msg, path) -> error_to_json ~path msg) errors in
         `Assoc [
+          "errors", `List errors;
           "data", data;
-          "errors", `List errors
         ]
 
   let to_response = function
@@ -1253,11 +1257,9 @@ end
     | Error (`Validation_error msg) ->
         Error (error_response msg)
     | Error (`Argument_error msg) ->
-        let `Assoc errors = error_response msg in
-        Error (`Assoc (("data", `Null)::errors))
+        Error (error_response ~data:`Null msg)
     | Error (`Resolve_error (msg, path)) ->
-        let `Assoc errors = error_response ~path msg in
-        Error (`Assoc (("data", `Null)::errors))
+        Error (error_response ~data:`Null ~path msg)
 
   let subscribe : type ctx. ctx execution_context -> ctx subscription_field -> Graphql_parser.field -> (Yojson.Basic.json response Io.Stream.t, [> resolve_error]) result Io.t
   =

graphql/test/argument_test.ml

@@ -68,12 +68,12 @@ let suite : (string * [>`Quick] * (unit -> unit)) list = [
   ("null for required argument", `Quick, fun () ->
     let query = "{ input_obj(x: null) }" in
     test_query query (`Assoc [
-      "data", `Null;
       "errors", `List [
         `Assoc [
           "message", `String "Argument `x` of type `person!` expected on field `input_obj`, found null."
         ]
-      ]
+      ];
+      "data", `Null;
     ])
   );
   ("missing optional argument", `Quick, fun () ->
@@ -87,12 +87,12 @@ let suite : (string * [>`Quick] * (unit -> unit)) list = [
   ("missing required argument", `Quick, fun () ->
     let query = "{ input_obj }" in
     test_query query (`Assoc [
-      "data", `Null;
       "errors", `List [
         `Assoc [
           "message", `String "Argument `x` of type `person!` expected on field `input_obj`, but not provided."
         ]
-      ]
+      ];
+      "data", `Null;
     ])
   );
   ("input coercion: single value to list", `Quick, fun () ->

graphql/test/error_test.ml

@@ -11,15 +11,15 @@ let suite = [
     ]) in
     let query = "{ nullable }" in
     test_query schema () query (`Assoc [
-      "data", `Assoc [
-        "nullable", `Null
-      ];
       "errors", `List [
         `Assoc [
           "message", `String "boom";
           "path", `List [`String "nullable"]
         ]
-      ]
+      ];
+      "data", `Assoc [
+        "nullable", `Null
+      ];
     ])
   );
   ("non-nullable error", `Quick, fun () ->
@@ -31,13 +31,13 @@ let suite = [
     ]) in
     let query = "{ non_nullable }" in
     test_query schema () query (`Assoc [
-      "data", `Null;
       "errors", `List [
         `Assoc [
           "message", `String "boom";
           "path", `List [`String "non_nullable"]
         ]
-      ]
+      ];
+      "data", `Null;
     ])
   );
   ("nested nullable error", `Quick, fun () ->
@@ -58,15 +58,15 @@ let suite = [
     in
     let query = "{ nullable { non_nullable } }" in
     test_query schema () query (`Assoc [
-      "data", `Assoc [
-        "nullable", `Null
-      ];
       "errors", `List [
         `Assoc [
           "message", `String "boom";
           "path", `List [`String "nullable"; `String "non_nullable"]
         ]
-      ]
+      ];
+      "data", `Assoc [
+        "nullable", `Null
+      ];
     ])
   );
   ("error in list", `Quick, fun () ->
@@ -91,6 +91,12 @@ let suite = [
     in
     let query = "{ foos { id } }" in
     test_query schema () query (`Assoc [
+      "errors", `List [
+        `Assoc [
+          "message", `String "boom";
+          "path", `List [`String "foos"; `Int 2; `String "id"]
+        ]
+      ];
       "data", `Assoc [
         "foos", `List [
           `Assoc [
@@ -104,12 +110,6 @@ let suite = [
           ];
         ]
       ];
-      "errors", `List [
-        `Assoc [
-          "message", `String "boom";
-          "path", `List [`String "foos"; `Int 2; `String "id"]
-        ]
-      ]
     ])
   );
 ]

graphql/test/schema_test.ml

@@ -270,13 +270,13 @@ let suite = [
   ("subscription returns an error", `Quick, fun () ->
     let query = "subscription { subscribe_to_user(error: true) { id name } }" in
     test_query query (`Assoc [
-      "data", `Null;
       "errors", `List [
         `Assoc [
           "message", `String "stream error";
           "path", `List [`String "subscribe_to_user"]
         ]
-      ]
+      ];
+      "data", `Null;
     ])
   );
   ("subscriptions: exn inside the stream", `Quick, fun () ->

graphql/test/variable_test.ml

@@ -82,12 +82,12 @@ let suite : (string * [>`Quick] * (unit -> unit)) list = [
     let variables = ["x", `Null] in
     let query = "{ input_obj(x: $x) }" in
     test_query variables query (`Assoc [
-      "data", `Null;
       "errors", `List [
         `Assoc [
           "message", `String "Argument `x` of type `person!` expected on field `input_obj`, found null."
         ]
-      ]
+      ];
+      "data", `Null;
     ])
   );
   ("variable coercion: single value to list", `Quick, fun () ->