nmcli: print to stderr where appropriate
Summary
Currently, nmcli logs a lot of error messages to stdout rather than stderr. This, at least in one case, results in junk being printed when making an offline connection configuration with --offline.
Fixes #1692
Tests needed fixing to accomodate for this, MR: NetworkManager-ci!1851
Purpose
Makes it so that stderr is used whenever it makes sense.
Checklist
Please read https://gitlab.freedesktop.org/NetworkManager/NetworkManager/-/blob/main/CONTRIBUTING.md before opening the merge request. In particular, check that:
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the subject for all commits is concise and explicative -
the message for all commits explains the reason for the change -
the source is properly formatted -
any relevant documentation is up to date -
you have added unit tests if applicable -
the NEWS file is updated when the change deserves to be mentioned, for example for new features, behavior changes, API deprecations, etc.