Prelude.undefined error message was introduced by
bb4f31a0ee.
It seems best to filter out local repositories that cannot be accessed
from the list of remotes, rather than keeping them in and making every
thing that uses the list have to deal with remotes that may have an unknown
location.
Besides fixing the error message, this also makes unavailable local
remotes' names not be shown in various messages, including in git annex
status output.
Also, move --to an unavailable local repository now avoids some ugly
errors like "changeWorkingDirectory: does not exist".
