Add descriptive error message to GetTypeName

Author: DJShinn

Optimizely.Graph.Source.Sdk/Optimizely.Graph.Source.Sdk/SourceConfiguration/SourceConfigurationModel.cs

@@ -255,7 +255,7 @@ private string GetTypeName(Type fieldType)
                 return "[String]";
             }
 
-            throw new NotImplementedException();
+            throw new NotImplementedException($"Property of type {fieldType.Name} is not assignable to any implemented types");
         }
     }