solver: Downgrade some warnings
solver: Downgrade some warnings

It's not always necessary to have critical warnings coming from the
solver.

By using `g_debug()` unconditionally we tap into the GLib logging
facilities, so it's always possible to get messages from the solver in
case something seems odd; additionally, development tools could show
debugging messages by default.

ebassi/emeus,ebassi/emeus,ebassi/emeus