confbridge: Minor fixes playing user counts to the conference.

* Generate a warning message if sound files do not exist when trying to
play the user count to the conference.  Use the new helper routine
sound_file_exists() for consistency.

* Put the new user into autoservice when playing user counts to the
conference.

* Check the return value of ast_bridge_impart().
........

Merged revisions 379808 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: http://svn.asterisk.org/svn/asterisk/trunk@379809 f38db490-d61c-443f-a65b-d21fe96a405b