Stop errors when changing an email address to an invalid one

We use the `ChangeEmailForm` if you want to change your own email
address or someone else's email address. This has various validators
which get run. We check if the email address is valid (by using a
function from utils) and if the email address is already in use
(by calling API).

If the email address is not valid, we should not call API to see if it's
already in use because this will cause an exception in API leading to a
`500` in admin. We now only call API if there were no other errors with
the email address.

(The `test_should_redirect_after_name_change` test didn't need the
`mock_email_is_not_already_in_use` fixture, so this has been removed.)

tests/app/main/views/test_user_profile.py

@@ -74,7 +74,6 @@ def test_should_show_name_page(
 def test_should_redirect_after_name_change(
     client_request,
     mock_update_user_attribute,
-    mock_email_is_not_already_in_use
 ):
     client_request.post(
         'main.user_profile_name',
@@ -109,6 +108,8 @@ def test_should_redirect_after_email_change(
         )
     )
 
+    assert mock_email_is_not_already_in_use.called
+
 
 @pytest.mark.parametrize('email_address,error_message', [
     ('me@example.com', 'Enter a public sector email address or find out who can use Notify'),
@@ -128,6 +129,8 @@ def test_should_show_errors_if_new_email_address_does_not_validate(
     )
 
     assert normalize_spaces(page.find('span', class_='govuk-error-message').text) == f'Error: {error_message}'
+    # We only call API to check if the email address is already in use if there are no other errors
+    assert not mock_email_is_not_already_in_use.called
 
 
 def test_should_show_authenticate_after_email_change(