STITCH-1354 Refactor error handling

[adamchel]

I tried to refactor errors to be as consistent and cohesive as possible. It’s not too many changes, but I did think a lot about it as well as all of the edge cases. I’m happy to clarify anything

The gist of the change is that there now only three types of errors ever thrown by the SDK:

• StitchClientError: errors thrown before a request is made
• StitchError: errors thrown after a request is completed and a response is received
• Transport-specific errors: errors thrown while a request is being carried out (timeouts, could not reach server, etc.)

Main changes:

• StitchError now only represents errors that are from completed requests.
  • We now have case unknownError in StitchError
  • We no longer have case requestError. Instances were either replaced with an appropriate StitchClientError, or with an appropriate unknownError .
• StitchClientError now only represents errors that can occur before a request is ever made.
• Transports are responsible for throwing all errors that may be thrown while carrying out the request, and this is clarified in the comments.
• We no longer directly throw any error codes.
• unknownError and invalidURL were removed from StitchErrorCode as they aren’t actually Stitch error codes and are now their own appropriate cases in the other error enums.
• If the error returned from the server can’t be parsed as a JSON ErrorCodable, Stitch will return an unknown error with the response body text as the error message.

Drive-bys:

• XCTAssertThrowsError was not checking error messages as we thought, so I fixed all of our usages of it to actually verify that the correct error was being thrown
  • In the case of the roundTrip errors, they weren’t actually throwing Stitch errors at all, they were throwing errors specific to the transport, which is what we want
• Removed usages of try! StitchCore.sync and replaced with objc_ stuff.

[adamchel]

Just modified the PR a little bit to address the concerns in the Slack discussion. There are now three types of errors, and all errors are under the singular StitchError enum. The comments in the StitchError enum describe the new structure.

[adamchel]

@edaniels @jsflax PTAL, along with the new Java PR.