Log error instead of panicking when child fails to start

[roderickvd]

librespot currently panics when onevent programs fail to start, which seems a bit harsh -- there's nothing in the way of resuming music streaming. This PR handles this gracefully.

[sashahilton00]

I believe we decided to make it panic originally as we assumed a valid, functioning program as a prerequisite, hence the panic meant that any error caused an immediate failure as opposed to continuing regardless. I'm indifferent as to whether it logs or panics, I can see arguments for both.

[Johannesd3]

IMHO, Rust programs should never panic. Panics are actually meant for logic errors, i.e. programmer's mistakes. The proper way is to propagate the error down to the main function, and terminate the program printing an error message.

[sashahilton00]

That's fair enough. Given that it spits out an error if the script fails, I think we can probably change this behaviour as I agree, panicking does feel heavy handed.

[roderickvd]

IMHO, Rust programs should never panic. Panics are actually meant for logic errors, i.e. programmer's mistakes. The proper way is to propagate the error down to the main function, and terminate the program printing an error message.

Is this we want to improve in general? There is a lot of .expect() and .unwrap() going on for operations that may well fail (e.g. incorrect command line parameters, failure to open an audio backend) that could be made nicer. It's not high priority but I'd be willing to invest some time in it.