[Feature]: Make callback-version of sendIq return AnyPromise

[tmolitor-stud-tu]

Precondition

• I understood that this template is only for feature requests and not for bug reports

Cross-checked related issues?

• I have cross-checked this overview Overview: Tasks and Issues #322 as well as filtered for related labels https://github.com/monal-im/Monal/labels

Describe your feature

Replace the success and error callbacks provided to xmpp.m's sendIq: let sendIq return AnyPromise instead.

To resolve that properly, any error IQ must conform to NSError, too. This is needed to make PromiseKit call the attached error handler of the promise instead of the success handler.