Expose mono_class_is_open_constructed_type to Unity to allow us to check to see if the class is not yet fully instantiated prior to asttempting to call mono_object_new on it.

* Also cleaning up a minor leak of a MonoError in mono_unity_class_inflate_generic_class

* mono_custom_attrs_construct_by_type allows unity to have a chance at handling the error if one arises instead of fatally asserting
* mono_unity_error_convert_to_exception allows unity to take the provided MonoError and fetch the MonoException object within to log into the editor as the exception that was thrown.

This change has associated unity changes in order to fix (case 1255935)

this to check if a class has a static constructor, so we can call it
before a static method gets invoked.

This was previously stubbed out when rebasing
onto mono master branch.

Change due the fact that the interface is soon to be used in other places.

Unity unloads domains and this is accomplished via thread abort exceptions.
Mono now matches .NET behavior by not interrupting catch/finally blocks with
thread abort exceptions. This can lead to hangs in Unity as domain unload
blocks until all thread are interrupted.

[unitytls] Added missing MONO_API to mono_unity_get_unitytls_interface and added it to the header for completeness

Add mono_custom_attrs_get_attrs API that was missed during rebase onto master. https://github.com/Unity-Technologies/mono/commit/1449b13fec64915a61b45612ae5c509d9c7a52f9