Better error reporting when an expected bridged method is not found

[marcprux]

The R8/proguard process for Android release builds will often minify (i.e., mangle) Java names. When the Swift side of the bridge attempts to look up a method name that cannot be found like so:

let Java_set_count_methodID = Java_class.getMethodID(name: "setCount", sig: "(I)V")!

the program will simply crash with the unhelpful error:

Fatal error: Unexpectedly found nil while unwrapping an Optional value

What we might instead do is have the method lookup happen in a forced try! with a throwable function, so the error log will at least contain the name of the method that was sought:

let Java_set_count_methodID = try! Java_class.getMethodIDAndThrowErrorIfMissing(name: "setCount", sig: "(I)V")

That way, we can get an error like:

Fatal error: Could not find Java method setCount on the class APIClient bridged from APIClient.swift:1234 (release mode; did you add it to your proguard-rules.pro file?)

This will at least provide some guidance to the user at development time (and in Crashlytics reports if it happens in a deployed app) about the potential cause of the method not being found.