sysupdate: Add a flag to control error behaviour in internal function
Optionally prevent `context_read_definitions()` erroring out if zero
transfer definitions were found.
This commit makes no functional changes (the flag is always passed to
calls to `context_make_offline()` for the moment), but the new flag will be
used in the following commit.
Signed-off-by: Philip Withnall <pwithnall@gnome.org>
Helps: https://github.com/systemd/systemd/issues/41501