Makes uses of the additions to utils in https://github.com/alphagov/notifications-utils/pull/9
This commit strips out a lot of the complex stuff that the views and templates
in this app were doing. There is now a cleaner separation of concerns:
- utils returns the number and type of errors in the csv
- `get_errors_for_csv` helper in this app maps the number and type of errors
onto human-friendly error messages
- the view and template just doing the glueing-together of all the pieces
This is (hopefully) easier to understand, definitely makes the component
parts easier to test in isolation, and makes it easier to give more specific
error messages.
