Handle missing external dependencies more graceful

[SpraxDev]

Right now binaries like ffmpeg and ffprobe (only an example) are required to be installed and in the PATH env.

Sometimes even crashing Apollo if it is missing. We should have checks in place if these external tools are available at app startup to not run them if possible.
And in general improve error handling, including but not limited to adding error handlers to stdout/-err/-in of child processes.