Do not raise exception if "avformat_find_stream_info" fails

Just ran into some files that were perfectly fine, and could be parsed without
problems, but still failed on avformat_find_stream_info() with AVERROR_NOTSUPP.
Removed raising the exception solved the problem. I have to see if this causes
problems long term, and if this call is needed at all.
Quick fix to compile with Ruby 2.0.0