[PATCH] Improve signer name error in `mml-secure-epg-sign'.

[Karl Fogel]

I hope someone with more expertise in this code can review and possibly improve 
this patch.

Recently I got this error message when trying to send a GPG-signed message:

  "Couldn't find any signer names.  Perhaps `mml-secure-smime-sign-with-sender' 
should be set?"

However, `mml-secure-smime-sign-with-sender' was already set to t (the actual 
problem lies elsewhere and I'm still debugging it).  Since Emacs can know that 
`mml-secure-smime-sign-with-sender' is set, there's no reason for the error 
message to send the user down the wrong road.  The attached patch 
conditionalizes the error message accordingly.

I believe this patch could be improved by offering the user more advice if a 
signer cannot be found.  For example, maybe the user should look at setting 
`mm-sign-option' to `guided'?  However, I am not an expert in this; my setup 
(with `mml-secure-smime-sign-with-sender' set to t) had been working fine for 
years and then suddenly broke, so I'm now debugging.  It's just that near the 
beginning of my debugging, Emacs presented me with a misleading error message 
-- hence this patch, so others can at least avoid that false start.  Further 
suggestions or improvements welcome.

Best regards,
-Karl

[[[
Improve an error message about message signer

* lisp/gnus/mml-sec.el (mml-secure-epg-sign): Don't suggest setting
  `mml-secure-smime-sign-with-sender' if it's already non-nil.
]]]

--- lisp/gnus/mml-sec.el
+++ lisp/gnus/mml-sec.el
@@ -945,7 +945,11 @@ mml-secure-epg-sign
         (signers (mml-secure-signers context signer-names))
         signature micalg)
     (unless signers
-      (error "Couldn't find any signer names.  Perhaps 
`mml-secure-smime-sign-with-sender' should be set?"))
+      (let ((maybe-msg
+             (if mml-secure-smime-sign-with-sender
+                 "."
+               "; try setting `mml-secure-smime-sign-with-sender'.")))
+        (error "Couldn't find any signer names%s" maybe-msg)))
     (when (eq 'OpenPGP protocol)
       (setf (epg-context-armor context) t)
       (setf (epg-context-textmode context) t)