fix(ffmpeg_producer): Prevent loading unreadable files

[ianshade]

Adds additional check if the file is readable while checking if it's valid. This prevents loading a file that is locked due to pending copy etc. Currently, an FFmpeg exception would be thrown, and even though it is handled gracefully, 202 is returned, making clients unaware of the failure. After the fix, 404 will be returned in such cases.