Make Guzzle exception handling forward-compatible with Guzzle 7

Problem/Motivation

In #3104353: Upgrade to Guzzle 7 we discovered that, aside from #3187315: Remove mink-goutte-driver as a core dependency, the only change required to make Drupal 9 compatible with Guzzle 7 is a small change to what exception we catch when handling request errors.

Both Guzzle\Exception\RequestException and Guzzle\Exception\ConnectException exist in both Guzzle 6 and 7. However, in Guzzle 6, ConnectException extends RequestException, whereas in Guzzle 7 they are siblings that implement different interfaces.

Both exceptions extend TransferException and are the only two types of TransferException within Guzzle itself. Guzzle itself also never calls TransferException directly. Furthermore, every usage we have of RequestException is catching it to log it, re-throw it as something else, set an error condition, etc. and so all those places are basically already allowing both RequestException and ConnectException since it's a child.

Proposed resolution

In most cases, catch TransferException instead. This will usually give us the same behavior unless the code specifically relies on the APIs of RequestException. In the latter case, we just need to add code to handle RequestException and Connectexception separately.

Remaining tasks

• MR incoming.
• It will need a CR, but I think it's non-disruptive enough that it doesn't need a release note.

API changes

Slight, backwards-compatible changes to Guzzle exception handling.

Release notes snippet

TBD if needed