make warnings during installation more obvious

[jluebbe]

Is your feature request related to a problem? Please describe.
Currently, if a minor issue is detected during installation, we can only log it on the service side, where it's easy to miss.
If warnings were more obvious, we could also use them to guide users away from deprecated features.

Describe the solution you'd like
After a successful rauc install (or the D-Bus alternative), any warning that occurred during installation should be printed.
One option would be to introduce a "Warnings" D-Bus property with a list of warnings, that is updated before the "Completed" signal is sent.

Warnings generated during service startup could persist until the first operation an be show on rauc status.