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?
Actually I think a more apt description would be: "In rare circumstances, Subliminal may log an error with no call site." (the error's not coming from `instruments`--it's our use of `UIALogger.logError` above).