mercurial.el: don't raise an error from find-file-hook
find-file() shouldn't fail even if unreadable .hg directory is exists.
Not enabling hg-mode is sufficient for that senario.
error: pretxncommit.nocommits hook failed: no commits allowed
abort: no commits allowed
transaction abort!
rollback completed
error: pretxncommit.nocommits hook failed: no commits allowed
abort: no commits allowed
transaction abort!
rollback completed