Add generic error type to deferred/promise typing

[jessevdk]

It would be nice if the IPromise and IDeferred interfaces would also type the error type of the promise/deferred. I wanted to check if this is something that would be considered. If so, I can make a PR (I have a working version of this locally).

[dylans]

@jessevdk sure, send over the PR and we'll review, thanks!

[jkieboom]

PR: https://github.com/SitePen/mayhem/pull/26