ENYO-3506: enable cancel button on all "build" steps

The cancel button was disables on the "starting: build" popup as cancellation was broken. We reenable it as ENYO-3506 will fix the broken cancellation.
ENYO-3506: Use a static error to signal bukld cancelled

The build steps are executed in sequence using an Async.waterfall call. When the build is cancelled, the waterfall sequence needs to be stopped. The proper way to do that is to call the next() function with an Error() as parameter. So far, when a build generates an Error(), it gets propagated at project level so that the UI can display an error message. When the error is a build cancel, the Error should not be propagated as no message needs to be displayed.
The "build cancel" error is defined as a statics to make that sure everywhere we cancel (can happen during any step) we use the same specific Error, which can be detected easily on lines 496-498.
ENYO-3506 correcting cancellation for getUserData step

Propagating the "next" pointer of the async.waterfall build to the getUserData and getToken function to be able to perform a next(Error). Getting the AJAX cancellation to abort the async.