better errors, type checking in event handlers

[minrk]

• raise TypeError immediately on wrong type in decorators (closes observe should raise with better error if wrong argument passed.  #221)
• allow any container in parse_notifier_name (closes observe should allow any iterable containers.  #215, closes don't use assert. #219)

[rmorshea]

If we move forward with #230 this will get covered, by parse_notifier_name in TraitEventHandlers, and a similar change to parse_notifier_name that I happened to include - I hadn't tagged to close #221 which is why this might not have shown up.

[minrk]

@rmorshea I’m going to merge this one since it’s small and addresses the problem, so we can get 4.3 out the door. Then we can move forward with #230 for 5.0.