deal with old repositories with non-encrypted creds

See 2f3c3aa01f for backstory about how a repo
could be in this state.

When decryption fails, the repo must be using non-encrypted creds. Note
that creds are encrypted/decrypted using the encryption cipher which is
stored in the repo, so the decryption cannot fail due to missing gpg keys
etc. (For !shared encryptiom, the cipher is iteself encrypted using some
gpg key(s), and the decryption of the cipher happens earlier, so not
affected by this change.

Print a warning message for !shared repos, and continue on using the
cipher. Wrote a page explaining what users hit by this bug should do.

This commit was sponsored by Samuel Tardieu.

Remote/Helper/Encryptable.hs

@@ -11,6 +11,7 @@ module Remote.Helper.Encryptable (
 	noEncryptionUsed,
 	encryptionAlreadySetup,
 	remoteCipher,
+	remoteCipher',
 	embedCreds,
 	cipherKey,
 	storeCipher,
@@ -93,21 +94,24 @@ encryptionSetup c = maybe genCipher updateCipher $ extractCipher c
                 -- remotes (while being backward-compatible).
 		[ "keyid", "keyid+", "keyid-", "highRandomQuality" ]
 
+remoteCipher :: RemoteConfig -> Annex (Maybe Cipher)
+remoteCipher = fmap fst <$$> remoteCipher'
+
 {- Gets encryption Cipher. The decrypted Ciphers are cached in the Annex
  - state. -}
-remoteCipher :: RemoteConfig -> Annex (Maybe Cipher)
-remoteCipher c = go $ extractCipher c
+remoteCipher' :: RemoteConfig -> Annex (Maybe (Cipher, StorableCipher))
+remoteCipher' c = go $ extractCipher c
   where
 	go Nothing = return Nothing
 	go (Just encipher) = do
 		cache <- Annex.getState Annex.ciphers
 		case M.lookup encipher cache of
-			Just cipher -> return $ Just cipher
+			Just cipher -> return $ Just (cipher, encipher)
 			Nothing -> do
 				showNote "gpg"
 				cipher <- liftIO $ decryptCipher encipher
 				Annex.changeState (\s -> s { Annex.ciphers = M.insert encipher cipher cache })
-				return $ Just cipher
+				return $ Just (cipher, encipher)
 
 {- Checks if the remote's config allows storing creds in the remote's config.
  -