[core/input] Ignore errors during callbacks
[core/input] Ignore errors during callbacks

If a callback command cannot be executed, do not terminate the whole
bar. Furthermore, if such a failure occurs, try to fall back to a
globally (i.e. non widget-specific) command, so that callbacks
registered in the engine itself (such as mouse-wheel scrolling) still
function properly.

fixes #166

tobi-wan-kenobi/bumblebee-status,tobi-wan-kenobi/bumblebee-status