Enable Python execute() to return Triton error code

[kthui]

Related PRs:
triton-inference-server/server#6228
#300

Currently, the execute() can return an error if needed, for example:

error = pb_utils.TritonError("error message")

The pb_utils.TritonError() has a new optional parameter code that allows returning a code, for example:

error = pb_utils.TritonError(message="error message", code=pb_utils.ErrorCode.TRITONSERVER_ERROR_INVALID_ARG)