- warn rather than fail when specified bitrate need not be used
- fail with a useful message when specified bitrate is too low
- print potentially useful information if verbose

patch based mostly on suggestions from Michael Niedermayer and Loren Merritt


git-svn-id: file:///var/local/repositories/ffmpeg/trunk@6176 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b