Make single error instance of Error to prevent [object Object] error messages

src/stitching/errors.ts

@@ -107,7 +107,7 @@ export function checkResultAndHandleErrors(
     // If there is only one error, which contains a result property, pass the error through
     const newError =
       result.errors.length === 1 && hasResult(result.errors[0])
-        ? result.errors[0]
+        ? Object.setPrototypeOf(result.errors[0], Error.prototype)
         : new CombinedError(concatErrors(result.errors), result.errors);
     throw locatedError(newError, info.fieldNodes, responsePathAsArray(info.path));
   }

src/test/testErrors.ts

@@ -99,5 +99,23 @@ describe('Errors', () => {
         });
       }
     });
+
+    it('persists single error that is instance of Error if the initial error has path and extensions and is instance of Object', () => {
+      const result = {
+        errors: [
+          {
+            message: 'Test error',
+            extensions: {},
+            path: [] as any,
+          } as GraphQLError
+        ]
+      };
+      try {
+        checkResultAndHandleErrors(result, {} as GraphQLResolveInfo, 'responseKey');
+      } catch (e) {
+        assert.equal(e.message, 'Test error');
+        assert.instanceOf(e, Error);
+      }
+    });
   });
 });