Give better errors in signing failures in Gnus

* lisp/gnus/mml1991.el (mml1991-epg-sign): Give better error
messages when signing fails (bug#26298).

* lisp/gnus/mml2015.el (mml2015-epg-sign): Ditto.

lisp/gnus/mml1991.el

@@ -275,6 +275,8 @@ Whether the passphrase is cached at all is controlled by
 	(mm-decode-content-transfer-encoding cte)))
     (let* ((pair (mml-secure-epg-sign 'OpenPGP 'clear))
 	   (signature (car pair)))
+      (unless (stringp signature)
+        (error "Signature failed"))
       (delete-region (point-min) (point-max))
       (insert
        (with-temp-buffer

lisp/gnus/mml2015.el

@@ -958,6 +958,8 @@ If set, it overrides the setting of `mml2015-sign-with-sender'."
     (let* ((pair (mml-secure-epg-sign 'OpenPGP t))
 	   (signature (car pair))
 	   (micalg (cdr pair)))
+      (unless (stringp signature)
+        (error "Signature failed"))
       (goto-char (point-min))
       (insert (format "Content-Type: multipart/signed; boundary=\"%s\";\n"
 		      boundary))