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.
adding foo
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
pushing to ../2
searching for changes
adding changesets
adding manifests
adding file changes
added 1 changesets with 1 changes to 1 files