Remote: Set an error when a remote cannot be found.

Inside `git_remote_load`, the calls to `get_optional_config` use
`giterr_clear` to unset any errors that are set due to missing config
keys. If neither a fetch nor a push url config was found for a remote,
we should set an error again.
Merge pull request #2389 from arthurschreiber/arthur/set-error-when-no-remote-found

Remote: Set an error when a remote cannot be found.