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.

debian/changelog

@@ -3,6 +3,9 @@ git-annex (5.20140916) UNRELEASED; urgency=medium
   * Security fix for S3 and glacier when using embedcreds=yes with
     encryption=pubkey or encryption=hybrid. 
     The creds embedded in the git repo were *not* encrypted.
+    git-annex enableremote will warn when used on a remote that has
+    this problem. For details, see:
+    https://git-annex.branchable.com/upgrades/insecure_embedded_creds/
   * assistant: Detect when repository has been deleted or moved, and
     automatically shut down the assistant. Closes: #761261
   * Windows: Avoid crashing trying to list gpg secret keys, for gcrypt