Tiltfile: run local command and get exit code

A few times this week, I've written Tiltfile snippets for ppl that run a local command and check if that command succeeded or failed, e.g.:

def file_exists(f):
    code = local('test -f {}; echo $?'.format(f))
    # exit code 0 means `test` succeeded, i.e. file exists
    return str(code).strip()=='0'

Notably, we do NOT want to stop Tiltfile execution if the command fails.

Might be nice to have a builtin for local_with_exit_code (or similar) that will do this for us, i.e. run the command and return its exit code (WITHOUT stopping Tiltfile execution on a failed command).

Open question: would we still want this func to return the command's stdout?

Start by tracing the existing Tiltfile local command entry point and its behavior when a command fails. Define how a local_with_exit_code-style builtin should expose the exit code without stopping Tiltfile execution, and resolve whether stdout is also returned; done means the requested failure handling and return value are covered by tests.