Bad event handler templates bring the suite down.

[hjoliver]

E.g.:

[runtime]
    [[foo]]
         script = /bin/false
         [[[events]]]
               failed handler = echo %(nam)s  # "name"!

At runtime:

...
	  File "/home/vagrant/cylc.git/lib/cylc/task_events_mgr.py", line 842, in _setup_custom_event_handlers
	    "suite_url": quote(self.suite_url),
	KeyError: 'nam'
	
2017-07-07T13:44:27+10 ERROR - error caught: cleaning up before exit
2017-07-07T13:44:27+10 INFO - Suite shutting down - ERROR: 'nam'
2017-07-07T13:44:34+10 INFO - DONE

We should catch the exception, print a warning, and carry on.

[matthewrmshin]

I suppose we should catch this at validation?

[hjoliver]

That would be even better.

[matthewrmshin]

This will have to wait until after #2349 - we need to check the settings in [[[meta]]] for a full list of keys.

[matthewrmshin]

Fix to original issue proposed #2352. Validation to be done after #2349.