Only output a log message saying the format does not exist if it actually does...

Only output a log message saying the format does not exist if it actually does not exist, not if the file itself could not be opened.
(closes issue #11828)
Reported by: IgorG
Patches:
      readfile.v1.diff uploaded by IgorG (license 20)


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@103827 65c4cc65-6c06-0410-ace0-fbb531ad65f3