Give specific error if international letters off

If a service doesn’t have permission to send international letters but
someone tries to upload a letter with a valid international address we
just tell them that the last line must be a UK postcode.

This is a bit opaque and:
- suggests that we’re not recognising at all that it’s not a UK letter
- doesn’t explain why it must be a UK postcode

This commit adds a new, error message which tells users why their letter
can’t be sent. And hopefully will give them a better idea of how to
resolve the problem, if they really do need to be able to send
international letters.

app/main/forms.py

@@ -1252,6 +1252,10 @@ class LetterAddressForm(StripWhitespaceForm):
                 raise ValidationError(
                     f'Last line of the address must be a UK postcode or another country'
                 )
+            if address.international:
+                raise ValidationError(
+                    f'You do not have permission to send letters to other countries'
+                )
             raise ValidationError(
                 f'Last line of the address must be a real UK postcode'
             )