Improve the error message following content review.

'Session expired' or similar makes it sound like a new error.
It could confuse the user and make them think the sign in didn't work
and that their session has expired again.

So we went with:
The change you made was not saved. Please try again.

app/main/views/service_settings.py

@@ -113,7 +113,7 @@ def service_name_change(service_id):
 @user_has_permissions('manage_service')
 def service_name_change_confirm(service_id):
     if 'service_name_change' not in session:
-        flash("Session expired. Try again", 'error')
+        flash("The change you made was not saved. Please try again.", 'error')
         return redirect(url_for('main.service_name_change', service_id=service_id))
 
     # Validate password for form

tests/app/main/views/test_service_settings.py

@@ -717,7 +717,7 @@ def test_service_name_change_confirm_handles_expired_session(
     mock_verify_password.assert_not_called()
     mock_update_service.assert_not_called()
 
-    assert page.find('div', 'banner-dangerous').text.strip() == "Session expired. Try again"
+    assert page.find('div', 'banner-dangerous').text.strip() == "The change you made was not saved. Please try again."
 
 
 @pytest.mark.parametrize('volumes, consent_to_research, expected_estimated_volumes_item', [