MBS-12930: Validate relationship fields on Perl level

We were showing an error for this on the JS level, but
(because the submit button is a buggy knockout mess and wasn't disabled)
it was still possible to try to submit a seeded relationship
without selecting a type, which would cause an ISE.
Now it properly detects that in fact something is missing and blocks
it from submitting.

I don't think the error here is currently being shown on the browser,
but since it's the same string as for the JS one, it should be fine.

lib/MusicBrainz/Server/Form/Field/Relationship.pm

@@ -2,8 +2,8 @@ package MusicBrainz::Server::Form::Field::Relationship;
 use strict;
 use warnings;
 
-
 use HTML::FormHandler::Moose;
+use MusicBrainz::Server::Translation qw( l );
 extends 'HTML::FormHandler::Field::Compound';
 
 has_field 'relationship_id' => (
@@ -87,4 +87,15 @@ sub is_empty {
     return 0;
 }
 
+after 'validate' => sub {
+    my $self = shift;
+    my $link_type = $self->field('link_type_id')->value;
+
+    if (!$link_type) {
+        return $self->add_error(l('You must select a relationship type and target entity for every relationship.'));
+    }
+
+    return 1;
+};
+
 1;