Handle exception is org name already exists

Previously this would return a 500 error, as the 400 exception was
not handled from the API [1]. Note that:

- We tend to rely on exception messages to identify the error that
occurred [2][3], with services being a notable deviation [4]. I've
used the exception message approach, as this is more granular and
broadly consistent with the rest of the app.

- There is already code to cover this scenario when a user changes
the name of an existing organisation or service, but the mechanism
is different [5][6]. It makes sense to just get any error from the
call to try and create the organisation.

- The API mock is based on one for services [7], but I've chosen to
have it inline with the test, since we're unlikely to reuse it, and
it's clearer to have the test setup as part of the test.

[1]: 8f99da525d/app/organisation/rest.py (L34-L47)
[2]: 70b606a2d4/app/main/views/manage_users.py (L166)
[3]: 70b606a2d4/app/main/views/templates.py (L499)
[4]: 70b606a2d4/app/main/views/add_service.py (L30)
[5]: 70b606a2d4/app/main/views/service_settings.py (L102-L104)
[6]: 70b606a2d4/app/main/views/organisations.py (L264-L266)
[7]: 0abc143147/tests/conftest.py (L590-L606)

app/main/views/organisations.py

@@ -62,10 +62,17 @@ def add_organisation():
     form = NewOrganisationForm()
 
     if form.validate_on_submit():
-        return redirect(url_for(
-            '.organisation_settings',
-            org_id=Organisation.create_from_form(form).id,
-        ))
+        try:
+            return redirect(url_for(
+                '.organisation_settings',
+                org_id=Organisation.create_from_form(form).id,
+            ))
+        except HTTPError as e:
+            msg = 'Organisation name already exists'
+            if e.status_code == 400 and msg in e.message:
+                form.name.errors.append("This organisation name is already in use")
+            else:
+                raise e
 
     return render_template(
         'views/organisations/add-organisation.html',

tests/app/main/views/organisations/test_organisations.py

@@ -194,6 +194,37 @@ def test_create_new_organisation_fails_with_incorrect_input(
     assert error_message in page.select_one('.govuk-error-message').text
 
 
+def test_create_new_organisation_fails_with_duplicate_name(
+    client_request,
+    platform_admin_user,
+    mocker,
+):
+    def _create(**_kwargs):
+        json_mock = Mock(return_value={'message': 'Organisation name already exists'})
+        resp_mock = Mock(status_code=400, json=json_mock)
+        http_error = HTTPError(response=resp_mock, message="Default message")
+        raise http_error
+
+    mocker.patch(
+        'app.organisations_client.create_organisation',
+        side_effect=_create
+    )
+
+    client_request.login(platform_admin_user)
+    page = client_request.post(
+        '.add_organisation',
+        _data={
+            'name': 'Existing org',
+            'organisation_type': 'local',
+            'crown_status': 'non-crown',
+        },
+        _expected_status=200,
+    )
+
+    error_message = 'This organisation name is already in use'
+    assert error_message in page.select_one('.govuk-error-message').text
+
+
 @pytest.mark.parametrize('organisation_type, organisation, expected_status', (
     ('nhs_gp', None, 200),
     ('central', None, 403),