be more integrator-friendly

The functions in utils.py often implicitly do a `sys.exit(1)` (_error()...)

This is unkind to those that'd want to use these imports. Default to returning an empty string or false or something, then have the cli detect the return and exit for us...

Start by reading utils.py and tracing _error() and the other functions that can call sys.exit(1). Then inspect the CLI paths that use these functions to define how failures should be detected and reported. Done means imported functions no longer terminate the process implicitly, while the CLI still exits on those failures.

Written by the indexing model from the issue text.