Specify what happens when evaluating a categorical function input that's not in the domain

[jfirebaugh]

E.g. what does {"type": "categorical", "property": "p", "stops": [["foo", 0]]} evaluate to for the property value "bar"?

[davidtheclark]

I think this issue and #480 are vital for making categorical functions seem "usable". I'm not sure I'd feel very comfortable shipping a public DDS UI in Studio without them. (cc @mapbox/appbox)

[lucaswoj]

This issue was moved to mapbox/mapbox-gl-js#4144