fix($http): avoid `Possibly Unhandled Rejection` error when the request fails

Calling `promise.finally(...)` creates a new promise. If we don't return this promise, the user
won't be able to attach an error handler to it and thus won't be able to prevent a potential PUR
error.

This commit also improves the test coverage for the increment/decrement `outstandingRequestCount`
fix.

Closes #13869
Closes #14921