Merge pull request #2814 from lambdageek/dev/monoerror-appdomain

Merge pull request #2814 from lambdageek/dev/monoerror-appdomain

[appdomain] Cleanup MonoError code

1. Assert in mono_get_corlib_version instead of raising an exn.
2. Don't raise in mono_domain_create_appdomain external API function.
3. Assert in mono_domain_assembly_postload_search instead of raising.
4. Don't raise in mono_domain_fire_assembly_load.
5. Use mono_error_set_pending_exception in
   ves_icall_System_AppDomain_InternalGetProcessGuid
6. Dont' raise in unload_thread_main, populate unload_data instead.