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