Add catch to promise objects

[tracker1]

This relates to #1722 but a far simpler request... given that the .then method matches the Promise spec (with the extra progress parameter), it would be nice that there be a compatible .catch method... (per @dmethvin) this could call and return .then(null,fn) as a passthrough.

[dmethvin]

I can see us creating .catch(fn) as an alias for .then(null, fn). That is different than .done(fn) though because the latter adds a new handler to the existing Deferred.

[tracker1]

of course, this would need to be done with ['catch'] so as not to trigger reserved word treatment in older browsers.

[dcherman]

Giant +1 on this. Since these Promises aren't based on a prototype, it's much easier for this to be supported internally rather than having to do something like wrap the constructor and decorate the returned promise.

[gibson042]

I updated the title for accuracy. Suggested implementation:

"catch": function( fn ) {
    return promise.then( null, fn );
}

[markelog]

Sounds like a blocker to me