vcs: tweak how revisions and repo names are shown in error messages

Decode bytes to str, and show repo name instead of repr or full server file
system path. In some places, it will only report the "basename" of the
repository, without any "group names" that also would be nice to have. The easy
alternative would be to show the full file system path ... but it would be
unfortunate to leak absolute server side paths to end users.