better probing for gcrypt repositories using new --check option

Now can tell if a repo uses gcrypt or not, and whether it's decryptable
with the current gpg keys.

This closes the hole that undecryptable gcrypt repos could have before been
combined into the repo in encrypted mode.

Assistant/WebApp/Configurators/Local.hs

@@ -307,24 +307,21 @@ getFinishAddDriveR drive = go
 		, page "Encrypt repository" (Just Configuration) $
 			$(widgetFile "configurators/needgcrypt")
 		)
-	{- Either making a new unencrypted repo, or combining with
-	 - an existing unencrypted repo, or combining with an existing
-	 - gcrypt special remot, or some other existing gcrypt repo. -}
 	go NoRepoKey = do
-		mu <- liftIO $ probeGCryptRemoteUUID dir
-		case mu of
-			Just u -> enablegcryptremote u
-			Nothing ->
-				ifM (liftAnnex $ inRepo $ Git.GCrypt.probeGCryptRepo dir)
-					( error "The drive contains a gcrypt repository that is not a git-annex special remote. This is not supported."
-					, makeunencrypted
-					)
-	{- Sync the git-annex branch from the gcrypt repo, in order to
-	 - make sure we know how the special remote should be set up. -}
+		pr <- liftAnnex $ inRepo $ Git.GCrypt.probeRepo dir
+		case pr of
+			Git.GCrypt.Decryptable -> do
+				mu <- liftIO $ probeGCryptRemoteUUID dir
+				case mu of
+					Just u -> enablegcryptremote u
+					Nothing -> error "The drive contains a gcrypt repository that is not a git-annex special remote. This is not supported."
+			Git.GCrypt.NotDecryptable ->
+				error $ "The drive contains a git repository that is encrypted with a GnuPG key that you do not have."
+			Git.GCrypt.NotEncrypted -> makeunencrypted
 	enablegcryptremote u = do
 		mname <- liftAnnex $ getGCryptRemoteName u dir
 		case mname of
-			Nothing -> error $ "Unable to use the gcrypt remote at " ++ dir ++ ". Perhaps it is encrypted using a GnuPG key that you do not have?"
+			Nothing -> error $ "Cannot find configuration for the gcrypt remote at " ++ dir
 			Just name -> makewith $ const $ do
 				r <- liftAnnex $ addRemote $
 					enableSpecialRemote name GCrypt.remote $ M.fromList