Block all three code paths in message_send_chat_pgp that previously
fell back to sending unencrypted messages: encryption failure, missing
PGP key, and missing LibGPGME. Each path now shows a specific error
in the chat window and returns NULL so the caller skips logging and
display.
Also replace the generic cons_show for OX signcrypt failure with a
win_println on the current window, ensuring the user sees the error
even when the chat window is not focused.
Improve p_gpg_encrypt error reporting by adding a gchar** err
out-parameter. Each failure path now sets a descriptive message
(e.g. missing recipient key, missing sender key, GPGME context
failure, encryption failure) so the caller can display the exact
reason to the user.
Fix a memory leak: gpgme_key_unref(receiver_key) was missing from
the sender_key failure path.
