Hello,

having multiple remotes, which are often unreachable, I get command output like:

    git annex drop .
    drop 2017 Haus Idstein/26108065_10212412239435248_1115645256_o.jpg (locking horus...) (lockcontent failed) (checking horus...) ssh: Could not resolve hostname horus.local: Name or service not known
    (checking S3...) ok
    drop 2017 Haus Idstein/26177516_10212412238475224_326557077_o.jpg (locking horus...) (lockcontent failed) (checking horus...) ssh: Could not resolve hostname horus.local: Name or service not known
    (checking S3...) ok
    drop 2017 Haus Idstein/26178378_10212412238835233_820819105_o.jpg (locking horus...) (lockcontent failed) (checking horus...) ssh: Could not resolve hostname horus.local: Name or service not known
    (checking S3...) ok

which takes a long time, because it tries to lock horus first for each, waits for timeout and then tries the next remote. Of course I could use ```git annex drop --from=S3``` but I do not always remember that. Is there a way to auto ignore a remote which is not reachable?

Thanks!
