revlog: avoid raising no-arg RevlogError for internal flow control
I'm about to make RevlogError require a `message` argument and this
code was failing. This patch refactors it to not raise an exception
for intra-function flow control.
Differential Revision: https://phab.mercurial-scm.org/D10740
$ LANG=nonexistent LC_ALL=nonexistent LANGUAGE=nonexistent hg version -q
Mercurial Distributed SCM (version *) (glob)