app.py: str(e) instead of "{}".format(e)

We're not implementing format anywhere, so there's no reason to prefer format() over the more conventional conversion with str().

In one case no conversion is needed at all, so remove that.

Note that SubprocessError is a vanilla subclass of Exception: https://github.com/python/cpython/blob/bafa8487f77fa076de3a06755399daf81cb75598/Lib/subprocess.py#L96 483a68d9