woob.exceptions: don't subclass BrowserUnavailable with ClientError

An 4XX error is most likely caused by a browsing error in the weboob module,
not by a temporary unavailability of the server. This commit pushes the changes
of 12736e1a32852bd16d33c59f8cc50d4ea1adf7ca further by considering all
4XX responses as a bug (in addition to 404 errors). This change is expected
to help bug monitoring by reducing the number of "fake" BrowserUnavailable
occurences.