Add debug mode, activated by setting the DEBUG_REMOTEHG env var

For now, all debug mode does is to let some exceptions propagate that
may occur when e.g. cloning a repository.
Additionally, we get rid of one try/except pair which replaced helpful
exception messages with a bland 'Repository error'; now the user can
e.g. distinguish whether a 404 error occurred, authentication failed
or the domain name is unknown (due to a typo).