Update force casts and force unwraps in backend impls to print custom messages on failure

[stackotter]

At the moment the force casts and force unwraps print relatively generic messages when they cause fatal errors. We should probably replace the force casts and force unwraps with helper methods that use #function, #line, and Self to emit nicer fatal error messages

Here's one idea for a way we could format the messages,

Fatal(AppKitBackend): NSTextView is not NSButton (`updateButton(_:label:environment:action:)`, line 593)

[stackotter]

Alternatively, we could update all AppBackend methods to be throwing and then emit failed casts as errors. That would allow for central handling of such errors. One use case for central handling of internal SwiftCrossUI failures could be automated bug report generation. Reports could be saved to disk along with a dump of the view graph that caused the failure, or something along those lines.