don't show sync failure for drive that's not plugged in

Assistant/Sync.hs

@@ -165,9 +165,10 @@ syncAction rs a
 	| otherwise = do
 		i <- addAlert $ syncAlert nonxmppremotes
 		failed <- a rs
+		let failed' = filter (Git.repoIsLocalUnknown . Remote.repo) failed
 		let succeeded = filter (`notElem` failed) nonxmppremotes
 		updateAlertMap $ mergeAlert i $
-			syncResultAlert succeeded failed
+			syncResultAlert succeeded failed'
 		return failed
   where
 	nonxmppremotes = filter (not . isXMPPRemote) rs