Raise a non-field error when neither or both link_page and link_url have been specified, instead of raising a field error for both fields.
Raise a non-field error when neither or both link_page and link_url have been specified, instead of raising a field error for both fields.

ababic/wagtailmenus,rkhleics/wagtailmenus,rkhleics/wagtailmenus,ababic/wagtailmenus,ababic/wagtailmenus,rkhleics/wagtailmenus