The new 'v2' API wants to return less data than the previous one,
which was sending back tons of fields the clients never used.
This new route returns only useful information, with the JSON
response dict being built up in the model's `.serialize()` method.
Note that writing the test for this was a bit painful because of
having to treat loads of keys differently. Hopefully we think this
is a good way to write this test, because if we don't, we should
start thinking of a better way to check the values are what we
expect.
In the V2 API, the GET response for an individual notification
returns a 'cost' value, which we can get by multiplying the
billable units by the per-message rate of the supplier who
sent the message.
Any notifications with billable units > 0 but without a
corresponding `ProviderRates` entry will blow up the application,
so make sure you've got one.
Update the format_checkers to raise the specific exception that why the validator can handle multiple messages.
Which led to a refactor of build_error_message.
