Add check for missing AssemblyNativeVersionAttribute (#60)

***NO_CI***

Author: josesimoes

source/MetadataProcessor.Console/Program.cs

@@ -146,10 +146,8 @@ public void GenerateSkeleton(
                 }
                 catch (Exception ex)
                 {
-                    System.Console.Error.WriteLine(
-                        "Unable to generate skeleton files");
-
-                    Environment.Exit(1);
+                    System.Console.Error.WriteLine("Unable to generate skeleton files.");
+                    throw;
                 }
             }
 

source/MetadataProcessor.Core/nanoSkeletonGenerator.cs

@@ -293,6 +293,15 @@ private void GenerateAssemblyLookup()
         {
             // grab native version from assembly attribute
             var nativeVersionAttribute = _tablesContext.AssemblyDefinition.CustomAttributes.FirstOrDefault(a => a?.AttributeType?.Name == "AssemblyNativeVersionAttribute");
+
+            // check for existing AssemblyNativeVersionAttribute
+            if (nativeVersionAttribute == null)
+            {
+                throw new ArgumentException("Missing AssemblyNativeVersionAttribute." + Environment.NewLine +
+                    "Make sure that AssemblyNativeVersionAttribute is defined in AssemblyInfo.cs, like this:" + Environment.NewLine +
+                    "[assembly: AssemblyNativeVersion(\"1.0.0.0\")]");
+            }
+
             Version nativeVersion = new Version((string)nativeVersionAttribute.ConstructorArguments[0].Value);
 
             var assemblyLookup = new AssemblyLookupTable()