Duplicated error message when adding invalid connection string #921

As shown in the screenshot, the same error message can be triggered twice by clicking both "Test connection" and "Add data source".

Steps to reproduce:

1. Create a new knowledge base, or use an existing one.
2. Go to the data source configuration (/admin?tab=datasources)
3. Click "+ Add data source"
4. On the "Engine" dropdown field, select "Connection string"
5. Add any text to the field "name" (this, alongside the following step, enable the "Add data source" button)
6. On the field "connection string", add a string with an unrecognized engine prefix (in the screenshot, I've used "na://")
7. Click both "Test connection" and "Add data source", in any order.