Improve an error about the 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.
1 job for master in 82 minutes and 16 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Test
failed #4165
test-all

01:22:16

 
Name Stage Failure
failed
test-all Test
Makefile:319: recipe for target 'check-doit' failed
make[2]: *** [check-doit] Error 1
make[2]: Leaving directory '/builds/emacs/emacs/test'
Makefile:294: recipe for target 'check-expensive' failed
make[1]: *** [check-expensive] Error 2
make[1]: Leaving directory '/builds/emacs/emacs/test'
Makefile:959: recipe for target 'check-expensive' failed
make: *** [check-expensive] Error 2
ERROR: Job failed: exit code 1