Smallfix; add developer-friendly warning for missing modeladmin
Smallfix; add developer-friendly warning for missing modeladmin

Found while writing modeladmin tests

pombredanne/django-fluent-contents,django-fluent/django-fluent-contents,edoburu/django-fluent-contents,edoburu/django-fluent-contents,django-fluent/django-fluent-contents,edoburu/django-fluent-contents,jpotterm/django-fluent-contents,django-fluent/django-fluent-contents,ixc/django-fluent-contents,pombredanne/django-fluent-contents,jpotterm/django-fluent-contents,ixc/django-fluent-contents,ixc/django-fluent-contents,jpotterm/django-fluent-contents,pombredanne/django-fluent-contents