Add configuration and data errors (#23)

Workflows can fail if either the configuration is not valid or they are
missing data in the webhook payload. New error variants have been added
for both cases.

src/workflow.rs

@@ -11,17 +11,25 @@ pub trait Workflow: Debug + Sync + Send {
     async fn process(&self, event: Event) -> Result<serde_json::Value, WorkflowError>;
 }
 
-#[derive(Clone, Eq, PartialEq, Ord, PartialOrd, Hash, Debug, Error)]
+#[derive(Debug, Error)]
 pub enum WorkflowError {
+    #[error("configuration was not valid")]
+    Configuration,
+
     #[error("{0}")]
-    Unknown(String),
+    MissingData(String),
+
+    #[error(transparent)]
+    UnexpectedError(#[from] anyhow::Error),
 }
 
 impl IntoResponse for WorkflowError {
     fn into_response(self) -> Response {
         match self {
-            WorkflowError::Unknown(error) => {
-                (StatusCode::INTERNAL_SERVER_ERROR, error).into_response()
+            WorkflowError::Configuration => StatusCode::OK.into_response(),
+            WorkflowError::MissingData(error) => (StatusCode::BAD_REQUEST, error).into_response(),
+            WorkflowError::UnexpectedError(error) => {
+                (StatusCode::INTERNAL_SERVER_ERROR, error.to_string()).into_response()
             }
         }
     }