handler events list not validated

[hjoliver]

For generic event handlers, the handler events list (in task config) is not checked for validity. We should have a list of all valid event names held in one place, in task_events_mgr.py, and then simply use that as an option list in the suite config spec.

[matthewrmshin]

Note: task event handlers can now handle custom task outputs, so this is a bit more complex.