Wrap every call to a user-overridable function in jobtypes in a try block

[guidow]

User-overridable functions in jobtypes may fail for various reasons. Wrap all calls to them in a try-block, so failures can be caught and dealt with appropriately.

[opalmer]

This is something I've been considering in the back of my head for a while now, thanks for opening up an issue. In general this should be decently easy to support so long as we can associate an unhandled exception with a task so we can post the error to the task itself. That should be fine though since we'll be inside of a job type which should have access to that information.