HTML-escape error messages (OVE-20160505-0012)

The instance in cgierror() is a potential cross-site scripting attack,
because an attacker could conceivably cause some module to raise an
exception that includes attacker-supplied HTML in its message, for
example via a crafted filename. (OVE-20160505-0012)

The instances in preprocess() is just correctness. It is not a
cross-site scripting attack, because an attacker could equally well
write the desired HTML themselves; the sanitize hook is what
protects us from cross-site scripting here.

IkiWiki.pm

@@ -1647,6 +1647,8 @@ sub preprocess ($$$;$$) {
 				if ($@) {
 					my $error=$@;
 					chomp $error;
+					eval q{use HTML::Entities};
+					$error = encode_entities($error);
 				 	$ret="[[!$command <span class=\"error\">".
 						gettext("Error").": $error"."</span>]]";
 				}

IkiWiki/CGI.pm

@@ -488,6 +488,9 @@ sub cgi (;$$) {
 sub cgierror ($) {
 	my $message=shift;
 
+	eval q{use HTML::Entities};
+	$message = encode_entities($message);
+
 	print "Content-type: text/html\n\n";
 	print cgitemplate(undef, gettext("Error"),
 		"<p class=\"error\">".gettext("Error").": $message</p>");