branch: error out if we cannot find the remote

branch: error out if we cannot find the remote

When we look for which remote corresponds to a remote-tracking branch,
we look in the refspecs to see which ones matches. If none do, we should
abort. We currently ignore the error message from this operation, so
let's not do that anymore.

As part of the test we're writing, let's test for the expected behaviour
if we cannot find a refspec which tells us what the remote-tracking
branch for a remote would look like.