MockLink tweaks to avoid uncaught exceptions

[hwillson]

Ensure MockLink (used by MockedProvider) returns all errors through the Link's Observable, instead of throwing some errors (like No more mocked responses for the query ...) leading to uncaught exceptions. All errors are now available via the error property of a result, and MockLink will no longer trigger any uncaught exceptions.

Fixes #6559

Please note: Returning all errors through the Link's Observable was the default behavior in Apollo Client 2.x. We changed it for 3, but the change has been problematic for those looking to migrate from 2.x to 3. We've decided to change this back with the understanding that not many people want or are relying on MockLink's throwing exception approach. If anyone is relying on this functionality, it can be re-created by leveraging MockLink.setOnError as follows:

const mocks = // ...your mocks...
const mockLink = new MockLink(mocks);
mockLink.setOnError(error => { throw error; });
// ... then call with something like
<MockedProvider link={mockLink}>

[benjamn]

@hwillson I think I see a way to make this less of a breaking change for people who already have code that uses setOnError. Let me know what you think!

[benjamn]

One final question: if we're passing errors thrown by onError to observer.error, I'm not sure the guidance we're giving in the CHANGELOG.md about setting mockLink.setOnError(error => { throw error }) will work, since that thrown error will just be passed to observer.error. I think that's a fine/good behavior, but perhaps we could simplify the recommendation to say setOnError can be used to customize the error handling?

[hwillson]

CHANGELOG updated @benjamn, so I think we're all set here. Thanks!