avoid error on missing compute state in checkKey

This improves eg `git-annex move --to` a compute remote that does not
contain the key. Rather than erroring with "Missing compute state" when
it checks if the key is in the remote, it proceeds to trying to store to
it, which has a nice error message.

Remote/Compute.hs

@@ -756,9 +756,7 @@ avoidCycles outputkeys inputkey = filterM go
 checkKey :: RemoteStateHandle -> Key -> Annex Bool
 checkKey rs k = do
 	states <- getComputeStatesUnsorted rs k
-	if null states
-		then giveup "Missing compute state"
-		else return True
+	return (not (null states))
 
 -- Unsetting the compute state will prevent computing the key.
 dropKey :: RemoteStateHandle -> Maybe SafeDropProof -> Key -> Annex ()