Poor error when unable to read the auth cookie
umoukun on irc points out that when your user can't read tor's auth cookie stem fails with...
stem.connection.UnreadableCookieFile: Authentication failed: '/var/run/tor/control.authcookie' doesn't existTrouble looks to be that _read_cookie() uses os.path.exists() which returns False if the file's not readable. We should perform a second permission check.
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