TS: Fix strict issues in src/error

src/__tests__/starWarsSchema.ts

@@ -98,7 +98,7 @@ const episodeEnum = new GraphQLEnumType({
  *     secretBackstory: String
  *   }
  */
-const characterInterface = new GraphQLInterfaceType({
+const characterInterface: GraphQLInterfaceType = new GraphQLInterfaceType({
   name: 'Character',
   description: 'A character in the Star Wars Trilogy',
   fields: () => ({

src/error/GraphQLError.ts

@@ -14,15 +14,6 @@ import { printLocation, printSourceLocation } from '../language/printLocation';
  * GraphQL document and/or execution result that correspond to the Error.
  */
 export class GraphQLError extends Error {
-  /**
-   * A message describing the Error for debugging purposes.
-   *
-   * Enumerable, and appears in the result of JSON.stringify().
-   *
-   * Note: should be treated as readonly, despite invariant usage.
-   */
-  message: string;
-
   /**
    * An array of { line, column } locations within the source GraphQL document
    * which correspond to this error.

src/error/__tests__/formatError-test.ts

@@ -45,10 +45,12 @@ describe('formatError: default error formatter', () => {
   });
 
   it('rejects null and undefined errors', () => {
+    // TODO ts-expect-error (formatError expects a value)
     expect(() => formatError(undefined)).to.throw(
       'Received null or undefined error.',
     );
 
+    // TODO ts-expect-error (formatError expects a value)
     expect(() => formatError(null)).to.throw(
       'Received null or undefined error.',
     );