If a remote has an empty url (which can happen for automatically created
remotes from some bundle files) we can't fetch from it. So for example
if you use "flatpak remote-ls --updates" it gives you a cryptic error
message from OSTree ("Failed to parse uri: "). This commit changes
flatpak to instead print an error message that mentions the remote name.
For backwards compatibility reasons, a remote with an empty url is
considered disabled, so say so.
Closes: #1165
Approved by: alexlarsson