Improve consistency: get rid of a zealous early error breaking apps in inconvenient ways

Since jQuery.on() and Backbone.Events.on are silently ignoring invalid callbacks,
Backbone.View.delegateEvents should too.

See #2184 for previous discussion about how Backbone is supposed to behave on obvious developer errors
Merge pull request #2191 from byroot/feature-ignore-undefined-methods-in-views-events

Improve consistency: get rid of "a zealous early error breaking apps in inconvenient ways"