tests/test-dirstate-backup.t

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

Set up

  $ hg init repo
  $ cd repo

Try to import an empty patch

  $ hg import --no-commit - <<EOF
  > EOF
  applying patch from stdin
  abort: stdin: no diffs found
  [10]

No dirstate backups are left behind

  $ ls .hg/dirstate* | sort
  .hg/dirstate