Give better error when sending to non-team member

Scenario we saw in research:
- trying to send a message to someone outside your team
- service is in trial mode

Result:
- error message was terrible, no-one understood it

Solution:
- better error message

app/notifications/rest.py

@@ -270,9 +270,13 @@ def send_notification(notification_type):
             [user.mobile_number, user.email_address] for user in service.users
         )
     ):
-        message = 'Invalid {} for restricted service'.format(first_column_heading[notification_type])
-        errors = {'to': [message]}
-        raise InvalidRequest(errors, status_code=400)
+        raise InvalidRequest(
+            {'to': [(
+                'Can’t send to this recipient when service is in trial mode '
+                '– see https://www.notifications.service.gov.uk/trial-mode'
+            )]},
+            status_code=400
+        )
 
     notification_id = create_uuid()
     notification.update({"template_version": template.version})