Fix error message generation (#10411)

* Fix error message generation

ApolloError.graphQLErrors is typed as an array of GraphQLError in typescript, but it can actually contain any value
at runtime, including null. This is even explicitly tested: https://github.com/apollographql/apollo-client/blob/caba6c191dcb7bee265d62aba131727c5536f4ef/src/core/__tests__/QueryManager/index.ts#L369-L376

This PR simplifies error message generation, and prevents ever creating the string 'undefined' as an error message.

See: #10394

* fix: handle protocolErrors

* chore: add changeset

---------

Co-authored-by: alessia <alessia@apollographql.com>

.changeset/heavy-carpets-chew.md

@@ -0,0 +1,5 @@
+---
+"@apollo/client": patch
+---
+
+Simplify error message generation and make 'undefined' an impossible message string.

src/errors/index.ts

@@ -2,7 +2,7 @@ import '../utilities/globals';
 
 import { GraphQLError, GraphQLErrorExtensions } from 'graphql';
 
-import { isNonEmptyArray } from '../utilities';
+import { isNonNullObject } from '../utilities';
 import { ServerParseError } from '../link/http';
 import { ServerError } from '../link/utils';
 import { FetchResult } from "../link/core";
@@ -51,31 +51,16 @@ export function isApolloError(err: Error): err is ApolloError {
 // If the error message has already been set through the
 // constructor or otherwise, this function is a nop.
 const generateErrorMessage = (err: ApolloError) => {
-  let message = '';
-  // If we have GraphQL errors present, add that to the error message.
-  if (
-    isNonEmptyArray(err.graphQLErrors) ||
-    isNonEmptyArray(err.clientErrors) ||
-    isNonEmptyArray(err.protocolErrors)
-  ) {
-    const errors = ((err.graphQLErrors || []) as readonly Error[])
-      .concat(err.clientErrors || [])
-      .concat((err.protocolErrors || []) as readonly Error[]);
-    errors.forEach((error: Error) => {
-      const errorMessage = error
-        ? error.message
-        : 'Error message not found.';
-      message += `${errorMessage}\n`;
-    });
-  }
-
-  if (err.networkError) {
-    message += `${err.networkError.message}\n`;
-  }
-
-  // strip newline from the end of the message
-  message = message.replace(/\n$/, '');
-  return message;
+  const errors = [
+    ...err.graphQLErrors,
+    ...err.clientErrors,
+    ...err.protocolErrors
+  ];
+  if (err.networkError) errors.push(err.networkError);
+  return errors
+    // The rest of the code sometimes unsafely types non-Error objects as GraphQLErrors
+    .map(err => isNonNullObject(err) && err.message || 'Error message not found.')
+    .join('\n');
 };
 
 export type GraphQLErrors = ReadonlyArray<GraphQLError>;