Merge pull request #2516 from lambdageek/monoerror-exception_instance

Merge pull request #2516 from lambdageek/monoerror-exception_instance

[runtime] Add MONO_ERROR_EXCEPTION_INSTANCE MonoError

This kind of MonoError can encapsulate a managed MonoException
object.  (For example as a result of mono_runtime_invoke ()).

Issues:

- [x] Memory leak correctness.
    `MonoError` already leaks memory if you simply swallow it without calling `mono_error_cleanup` or some function that calls it (for example `mono_error_raise_exception` or `mono_error_convert_to_exception`).
    Now if we have an encapsulated exception, we'll leak a GC handle unless cleanup happens.