In case of a 500 error, attempt a GET to determine if the URI might be incorrect

fix for https://github.com/doctrine/DoctrinePHPCRBundle/issues/154
my point is: if we check the login, we already see whether the server is misconfigured on login. so when a request later fails, there is no need to re-check as the login check would already have failed, so this can't be the problem.

if login check is disabled however, it makes sense to check on error if maybe the server is simply misconfigured to not have random errors.