Prevent people from uploading invalid podcast files

[markspolakovs]

If you upload an Audacity project file (.aup, common mistake) MyRadio will happily accept it as a podcast file. The transcoding step will fail, but since it's asynchronous you don't get any feedback (in fact I think we might be unconditionally marking the podcast as processed even if this fails...).

I see several areas for improvement:

• Sanity checks on the file (ffprobe maybe) before it's handed off for transcoding
• Better report transcode errors