lib/deploy: Use in-function error prefixing more
lib/deploy: Use in-function error prefixing more

Pulling some of this out of stage deploy work. It's generally better as it's
easier to change functions to have multiple callers.

Closes: #1505
Approved by: jlebon

GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree