brillo_update_payload: Show correct error when delta_generator not found.

If delta_generator is not in PATH, $(which delta_generator) will return 1
and then the script just go ahead to cleanup because trap is set on ERR,
but the error message never got a chance to be showed.

Bug: None
Test: run brillo_update_payload without delta_generator in PATH.

Change-Id: Ie7a6943f174f69e11ca795351d72c20516792f6f