Background panel spams journal while parsing dynamic wallpaper files

background: Don't warn for invalid xml tags

g_warning () is for environment errors. Additional tags than the ones we require are not necessarily an error.

This behavior causes g-c-c to spam the journal with entries such as:

Unknown Tag in /usr/share/gnome-background-properties/fedora-workstation-backgrounds.xml: license
Unknown Tag in /usr/share/gnome-background-properties/fedora-workstation-backgrounds.xml: email
Unknown Tag in /usr/share/gnome-background-properties/fedora-workstation-backgrounds.xml: author

Where "license", "email", and "author" are valid tags that we just don't use.