Mono: Add option to keep running after unhandled exceptions

By default, an unhandled exception will cause the application to be terminated; but the project setting `mono/unhandled_exception_policy` was added to change this behaviour.
The editor is hard-coded to never terminate because of unhandled exceptions, as that would make writing editor plugins a painful task, and we cannot kill the editor because of a mistake in a thirdparty plugin.

modules/mono/csharp_script.cpp

@@ -629,7 +629,6 @@ void CSharpLanguage::frame() {
 
 				if (exc) {
 					GDMonoUtils::debug_unhandled_exception(exc);
-					GD_UNREACHABLE();
 				}
 			}
 		}