Adding support for Throwable error result types in Kotlin

[emarteca]

More work on #697
Right now, if there is an error result passed from Rust to Kotlin, it is always thrown as a RuntimeException, which just calls the toString on the error. It is more useful to allow Kotlin to inspect the error. If the error result is represented by a Kotlin class, then we should be able to use the error result as an object of that class.
This also allows more finegrained error handling, instead of just having every error result from diplomat be a RuntimeException.

This PR adds support for returning structs and opaques as custom exceptions when they take the error position in a result. This is done by making all structs and opaques used in the error position of the result inherit from Exception -- then, they are Throwable, and in the .err() case on results the error result object itself can be thrown. Unfortunately this still doesn't handle the primitive error case.

Note: this PR also adds a new attribute, diplomat::attr(auto, error), that can be used to mark types. An error is thrown if non-type item is marked with this attribute. And, in the Kotlin backend, there is an error thrown if an opaque or struct not marked with this attribute is used in the error position in a result.

[Manishearth]

Hm, so what i'd prefer is we add a #[diplomat::attr(auto, error)] that can be applied to types. Thoughts?

(and then the Kotlin backend errors when you use non error types in Result)

(we'd add something similar for JS)

[Manishearth]

#446

[emarteca]

Oh yeah that's a good idea -- then, we could only add Throwable on the types that are actually marked with the error attribute. I'll do that.

Also, it doesn't look like Kotlin enums support inheritance, so I'm thinking we just have the error attribute requirement for structs and opaques for now. What do you think about (in a subsequent PR) adding a Throwable wrapper for all the primitives, and Throwable wrappers for enums marked as error?

[Manishearth]

Yeah, that would work!

[emarteca]

Ok I added the error attribute! Good idea.

I'll do a followup PR to add the Throwable wrappers for primitives, and for enums marked with diplomat::attr(auto,error), if that sounds good?

[jcrist1]

Looks nice!