When sending an encrypted response, send an extra plaintext one too

Just in case the user included the wrong public key, or doesn't know how to decrypt the email they received, the plaintext one should give them instructions on what to do.

No file or test is named. First locate the code path that sends encrypted email responses and read how response content and public-key failures are handled. Done means an encrypted response is accompanied by a plaintext response containing instructions for users who supplied the wrong key or cannot decrypt the message, with relevant tests updated or added.

Written by the indexing model from the issue text.