Add thesaurus from registry - empty error message if the url is not correct

[josegar74]

Login as administrator and go to the Administration > Classification systems and add a thesaurus with the option From registry

The option adds following path /registry/registry.en.json to the url to retrieve the list of vocabularies (for example http://inspire.ec.europa.eu/registry/registry.en.json), but if the url is not valid, for example includes already /registry a message is displayed, but empty. Should be displayed a meaningful message: