fix handling of ERROR in response to REMOVE

This allows an error message from a proxied special remote to be
displayed to the client.

In the case where removal from several nodes of a cluster fails,
there can be several errors. What to do? I decided to only show
the first error to the user. Probably in this case the user is not in a
position to do anything about an error message, so best keep it simple.
If the problem with the first node is fixed, they'll see the error from
the next node.

Remote/Git.hs

@@ -456,7 +456,7 @@ dropKey' repo r st@(State connpool duc _ _ _) key
 		)
 	| Git.repoIsHttp repo = giveup "dropping from http remote not supported"
 	| otherwise = P2PHelper.remove (uuid r) 
-		(Ssh.runProto r connpool (return (False, Nothing))) key
+		(Ssh.runProto r connpool (return (Right False, Nothing))) key
 
 lockKey :: Remote -> State -> Key -> (VerifiedCopy -> Annex r) -> Annex r
 lockKey r st key callback = do

Remote/Helper/P2P.hs

@@ -60,12 +60,14 @@ retrieve gc runner k af dest p verifyconfig = do
 			Just (False, _) -> giveup "Transfer failed"
 			Nothing -> remoteUnavail
 
-remove :: UUID -> ProtoRunner (Bool, Maybe [UUID]) -> Key -> Annex ()
+remove :: UUID -> ProtoRunner (Either String Bool, Maybe [UUID]) -> Key -> Annex ()
 remove remoteuuid runner k = runner (P2P.remove k) >>= \case
-	Just (True, alsoremoveduuids) -> note alsoremoveduuids
-	Just (False, alsoremoveduuids) -> do
+	Just (Right True, alsoremoveduuids) -> note alsoremoveduuids
+	Just (Right False, alsoremoveduuids) -> do
 		note alsoremoveduuids
 		giveup "removing content from remote failed"
+	Just (Left err, _) -> do
+		giveup (safeOutput err)
 	Nothing -> remoteUnavail
   where
 	-- The remote reports removal from other UUIDs than its own,