{db, repoconfmanager}.py: Improves error return handling

Previously repoconfmanager would not take into account that it
could get multiple return booleans from the different ConfigHandler
classes that it calls, this commit adds that awareness to the class
along with reflections of the change in db.py to interact with the
multiple returns that it might get from the RepoConfManager class.
cli.py: Fixes incorrect determination of supported overlay

For cli output of overlay info when running --info or --list, you
would previously get incorrect output telling you the overlay you
are listing isn't supported. This was due to the fact that the
boolean used to determine whether it was supported or not during
the unpacking of the overlay info was actually the boolean that
determined if the overlay was official or not.
repoconfmanager.py: modifies str checking

X-Gentoo-Bug: 518846
X-Gentoo-Bug-URL: https://bugs.gentoo.org/518846
cli.py: Removes unncessary comma placement

To add coherence to the layman output, messages have been altered
and unnecessay commas have been removed from the CLI messages.