unhandled exception ends up with error message indicating proto3_optional isn't supported

If there is an unhandled exception thrown by gapic-generator-typescript generating an API leveraging proto3_optional, the logs (at least in bazel) indicate that there was an error because the generator did not respond to protoc with the fact that it supports proto3_optional, see the following logs: https://screenshot.googleplex.com/4AQCdHQoJDU8uGS.

This might actually make sense because the plugin itself never "responds" to protoc (because it crashed), and protoc then logs that the plugin failed because it doesn't support proto3_optional.

This might actually need to be fixed in protoc, but perhaps the gapic-generator-typescript plugin runner should capture all exceptions, respond to protoc with an error response (and a proper flag indicating it supports proto3_optional) using the "unhandled" exception. I'm not sure, but it did add indirection to debugging this http://yaqs/5695674741042446336.