#2 Unhelpful error when trying to add a duplicate ISP

When adding a new ISP through a JSON URL, if the ISP already exists in the system, the error message is not at all helpful:

* Validating the JSON against the schema
/!\ Error with the validation API, connection was closed. Retry in a few seconds

The check for duplicate seems to come from WebValidator in crawler.py, but the error message is not propagated back to the user.

This issue can be closed, now error message is :

Error:This URL is already in our database

I managed to reproduce this issue by trying to submit the same project twice in two tabs. In normal circumstances it cannot happen because the duplicated json_url is detected on the previous page (https://db.ffdn.org/isp/create).