Don't crash when execute_method fails (#3104)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/3104

Currently, we hard crash the process when execute_method failed, and it's not catchable. Instead, we should return null to Java, so they can handle.

Reviewed By: shoumikhin, cccclai

Differential Revision: D56260831

fbshipit-source-id: 281aa53985e021e803444ea5ee1c89a1e4b66e6b