Revise error message for non-numeric responses

Things we talked about:
• asking users to write the number 'as numerals' or 'using digits' isn't
  very plain English
• the style guide says to use an example in the error `..., like 5,000`
  but not if you have an example in the hint text, so we can't do that
• I have reservations about 'correct format', because it sounds odd if
  you're not describing something like a phone number, NI number or
  credit card number.

Looking back through Request to Go Live tickets on Zendesk.
---

I got to September before I found anything that would count as invalid
under our new rules:

> Possibly around 1,000,000- not planning on implementing emails yet but
might change

I'll keep looking, but if most people enter the number according to the
hint example we might be able to go with a much simpler error just
prompting them to enter a number – no convoluted descriptions of what we
mean by a number

There seemed to be more problems when the Qs were about start volume and
peak volume. Users felt the need to explain their plans more.

Using 'number' instead of 'volume' is more explicit too – so that
probably helps.

In terms of errors:
`Enter the number of emails you expect to send`
`Enter the number of text messages you expect to send`
`Enter the number of letters you expect to send`
– will probably do it, right?

tests/app/main/views/test_service_settings.py

@@ -1000,7 +1000,7 @@ def test_should_not_default_to_zero_if_some_fields_dont_validate(
     ) == (
         'How many letters do you expect to send in the next year? '
         'For example, 50,000 '
-        'Number of letters must be in the correct format'
+        'Enter the number of letters you expect to send'
     )
     assert mock_update_service.called is False