[9.x] Http client: retry callback exception handling (follow-up to #41762)

[gdebrauwer]

Follow-up to #41762.

Because the retryWhenCallback is now executed inside the global retry helper method, an extra attempt would be made if an exception occurred in the retryWhenCallback. This was not the case when the retryWhenCallback was used as a callback by the global retry helper method.
By wrapping it in a try-catch, we can set $shouldRetry to false before we throw the exception again. This ensures it does not make an unnecessary attempt.