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.
Basic install:
$ make # see install targets
$ make install # do a system-wide install
$ hg debuginstall # sanity-check setup
$ hg # see help
See http://www.selenic.com/mercurial/ for detailed installation
instructions, platform-specific notes, and Mercurial user information.