This relationship is via the `Organisation` now; we don’t use this
column to fudge a relationship based on the user’s email address and the
matching something in these columns.
previously, it was too loose - checking `"name" in str(exc)` returns
false positives.
By changing from three if statements to a loop we can cut down on
unnecessary code (and ensure that the returned objects are consistent),
and by using the full check constraint name we can be sure that we're
only capturing exactly the right errors. Additionally, don't return
the original data in the error message - it's obvious what the name is
because it'll be populated in the form you just filled in.
However, until we can create a letter without a logo, we will still default to hm-government, because the dvla_organisation is set on the service.
This does simplify the code.
Also removed the inserts to letter_branding in the data migration file, because we can deploy this before the rest of the work is finished. But we will need to do it later.
