Fix crash(es) caused by disposed Rx observables.

[dbrant]

After Rx Observables are disposed, they can still produce errors, and if they are detached from an Observer when an error happens, the error goes to a "global" error handler defined in Rx. By default, this error handler does nothing, and allows the error to be unhandled, thereby causing a crash.

(I'm guessing you've seen these kinds of crashes -- it might happen right after you back out of an Activity, or anytime you dispose an observable that is still performing a network call.)

This fixes these crashes by providing an "empty" global Rx error handler. This means that any time you subscribe to an Observable, you should provide an onError handler (assuming you want to handle errors from it). And for Observables where you don't care about errors, they will simply fall through without crashing.

[dbrant]

Consolidated into #2669