cgierror: When the CGI fails, print the error to stderr, not "Died"

$@ could be clobbered by the "exception handler", and in practice
it seems that it is. This can be seen on stderr of t/git-cgi.t.

IkiWiki/CGI.pm

@@ -490,7 +490,8 @@ sub cgierror ($) {
 	print "Content-type: text/html\n\n";
 	print cgitemplate(undef, gettext("Error"),
 		"<p class=\"error\">".gettext("Error").": $message</p>");
-	die $@;
+
+	die $message;
 }
 
 1

debian/changelog

@@ -6,6 +6,7 @@ ikiwiki (3.20170112) UNRELEASED; urgency=medium
   * Guard against set-but-empty REMOTE_USER CGI variable on
     misconfigured nginx servers, and in general treat sessions with
     a set-but-empty name as if they were not signed in.
+  * When the CGI fails, print the error to stderr, not "Died"
 
  -- Simon McVittie <smcv@debian.org>  Sun, 14 May 2017 15:34:52 +0100