mw.api.postWithToken now forwards promise on fail if we have token

Without this fix the callers of this function could not read the
failure code and result, because we returned undefined. Because
we are using a filter function, we need to explicitly return the
promise to pass it forward.

Change-Id: I71984ac827618459f723b0b9f47d4dc29e2a2f70
Merge "mw.api.postWithToken now forwards promise on fail if we have token"