fix(updateengine): return AddMatch error

[SSOHEB]

What does this PR do?

Fixes incorrect error handling when registering the updateengine D-Bus signal match.

updateengine.New() currently checks call.Err after AddMatch(), but returns the earlier err variable instead. At this point err is nil because Hello() has already succeeded.

As a result, if AddMatch() fails, New() can return (nil, nil). The caller checks the returned error and proceeds with the nil client, which can subsequently be dereferenced.

This change returns the actual call.Err to the caller.

Testing done

• go test ./...
• git diff --check

All tests pass.

• Changelog entries added in the respective changelog/ directory (user-facing change, bug fix, security fix, update)
• Inspected CI output for image differences: /boot and /usr size, packages, list files for any missing binaries, kernel modules, config files, kernel modules, etc.

[tormath1]

Thanks.