Add better player tab crash handling

There is unfortunately no dedicated signal for this (the process API is for dev builds only), so we can only do:

• When we detect a tab becoming inaudible, check if it's gone now, and signal player gone. This will catch the case of a tab crashing while playing
• When sending a command, check if it failed, and signal player gone. Instead of doing nothing, the player will disappear. Not really better UX right there but at least the user will go "huh?" rather than "why is it not working?"

There's also no error codes on the error object, just sentences...

BUG: 419699

Differential Revision: https://phabricator.kde.org/D28614