Make form error messages consistent

We were using a bunch of different styles for form error messages,
including:
- having the name of the field in the error, or not
- can not/cannot/can’t (GDS content styleguide recommends using
  contractions)

tests/app/main/views/test_add_service.py

@@ -44,7 +44,7 @@ def test_should_return_form_errors_when_service_name_is_empty(app_,
             client.login(api_user_active, mocker)
             response = client.post(url_for('main.add_service'), data={})
             assert response.status_code == 200
-            assert 'Service name can’t be empty' in response.get_data(as_text=True)
+            assert 'Can’t be empty' in response.get_data(as_text=True)
 
 
 def test_should_return_form_errors_with_duplicate_service_name_regardless_of_case(app_,

tests/app/main/views/test_manage_users.py

@@ -260,7 +260,7 @@ def test_user_cant_invite_themselves(
         page = BeautifulSoup(response.data.decode('utf-8'), 'html.parser')
         assert page.h1.string.strip() == 'Invite a team member'
         form_error = page.find('span', class_='error-message').string.strip()
-        assert form_error == "You can't send an invitation to yourself"
+        assert form_error == "You can’t send an invitation to yourself"
         assert not mock_create_invite.called