When PATCH-ing a destination, the error message returned for is_duplicate is useless

[hmpf]

/../ The API does not behave correctly when PATCH-ing the email-address unchanged, it only complains about

{
  "non_field_errors": [
    "email_address"
  ]
}

.. stripping away the detail of the error for the email-address.

It would probably be better to instead return 200 Ok and pretend the PATCH never happened.

Originally posted by @hmpf in #936 (comment)

[hmpf]

A better way to show the error would be to have "non_field_errors" be a list of dictionaries:

{
  "non_field_errors": [
    { 
      "email_address": "This destination already exists for this user" 
    }
  ]
}