networks: Handle unknown connections gracefully
networks: Handle unknown connections gracefully

When trying to edit connection of unknown type show an error and return
to connections list instead of trying to filling in some fields and
doing badly.

jvalleroy/plinth-debian,jvalleroy/plinth-debian,harry-7/Plinth,vignanl/Plinth,kkampardi/Plinth,jvalleroy/plinth-debian,harry-7/Plinth,freedomboxtwh/Plinth,kkampardi/Plinth,freedomboxtwh/Plinth,kkampardi/Plinth,vignanl/Plinth,harry-7/Plinth,kkampardi/Plinth,jvalleroy/plinth-debian,freedomboxtwh/Plinth,harry-7/Plinth,kkampardi/Plinth,vignanl/Plinth,vignanl/Plinth,freedomboxtwh/Plinth,jvalleroy/plinth-debian,vignanl/Plinth,freedomboxtwh/Plinth,harry-7/Plinth