loader: Ensure NotFound exception is reraised when caught
Previous commit modified the dumb.check_protocol function to raise an HTTPError exception when the request to check dumb protocol support failed. As NotFound exception inherits from ValueError, the code for checking dumb protocol support was executed even when a repository was not found. So an HTTPError exception was raised with a 404 status code and the NotFound exception was no longer propagated to the base loader class, resulting in a failed visit status instead of a not_found one.
Fixes SWH-LOADER-GIT-1PN