Validate repository URLs and warn about and ignore bad repositories

[lfrancke]

Currently bad URLs will probably fail in get_repo_metadata when the URL is used for the first time.

That method currently has an expect with an empty string.

This issue contains two parts:

1. Validate each repository URL once as soon as it is downloads the list of repositories (see cannot_be_a_base among other things)
2. Just for completeness sake change the "expect" message to mention that/whay this can't fail