Handle exceptions from RACSignal+Operations blocks

[jspahrsummers]

For blocks given to RACSignal+Operations methods that don't themselves return a signal, it's difficult – if not outright impossible – to indicate error cases. RAC could catch exceptions thrown by those blocks (since it invokes them directly) and translate those into errors.

[joshaber]

If/when we do this, we should also support throwing exceptions within the lift methods.

[jspahrsummers]

There are some complexities around this that might make it untenable for 1.0. Exceptions and errors aren't trivially convertible in Cocoa (we'd probably need some special kind of error that includes the exception in its userInfo), and scheduling would probably have to be updated to be exception-safe.

I'd still like to do this at some point, but, given that exceptions are usually considered fatal in Cocoa, I don't think this is important enough to block a stable release on.

[joshaber]

👍

[jspahrsummers]

Meh.

[fatuhoku]

Hmm. I'm building a synchronization algorithm and as such BDD is pretty valuable. The process depends on accurate error feedback so that I can adjust the code in meaningful ways to make progress.

I opened #1546 because I ran into a situation where exceptions were being thrown and I had no way of finding out what was wrong.

[jspahrsummers]

As noted in #1546 (comment), exceptions are intended to be fatal in Objective-C. Catching them would cause all sorts of undefined behavior.