When an alert handler fails to run, log useful information

When an alert handler fails to run, log some useful information. When I added this, it appears subliminal-instrument would crash with an index out of bounds exception.

Thoughts?
Yeah, I think we might as well try to let the default handler get rid of the alert rather than abort the run (like `onAlert` does currently if the alert doesn't look like it can be dismissed at all). If it can't then the run's effectively over anyway.

I'd still splice out the handler, yeah. Each handler only gets one shot at dismissing a matching alert and this handler tried for this one.