Unexpected behaviour when using --index arg with non existant index name

[Kroppeb]

Running pipenv install --index=xxx should fail if xxx isn't a url and isn't a know index. Currently pipenv adds a source to the pipfile that has an empty string as name and just xxx as url, and subsequently fails to lock (without a clear error message).

We use a consistent certain name to refer to our internal package repo, and all our install commands listed on our internal readmes and wikis assume this one has been added to the sources list already. But if that hasn't happened, the error is very unclear on what went wrong.

Pipenv version: commit 6abb08c