networkmanager script failing, how to see full logs output
Solution 1:
it turns out it's NOT NetworkManager.service that would have the relevant logs:
a different service, NetworkManager-dispatcher.service, is what runs scripts in /etc/network/if-*.d/* - and the journalctl for that service does in fact have all stdout/stderr from the operation.
And for the curious, the underlying error was an exec format error from run-parts; the script we have generating the hooks had an extra newline i had not noticed:
# --> extra newline here, before ~shebang
#!/bin/sh
...