[ObjC] Improve error when binding contains non-simple not-exported types (such as BCL)

[shobhakartiwari]

error EM0009: The feature Returning type TextReader from native code is not currently supported by the tool

[chamons]

This appears to be the general "we don't support things that don't match a pattern we've listed" error:

https://github.com/mono/Embeddinator-4000/blob/master/objcgen/objcgenerator.cs#L1225

[chamons]

After doing some poking, I've verified this is the general "we only support limited return/param types" with classes with export. The HasClass \ HasProtocol check above is what we're failing.

This hits us if we try to use a Stream, or a other BCL types.

One could imagine even if this works, how would you use that TextReader, unless we also exposed every single method on it to ObjC as well.

I would suggest changing your API to return an object that contains the TextReader internally and exposes whatever items you need in simpler terms.

[chamons]

I'm going to convert this issue into a "we should drastically improve the error message".