pygmentize.pl: warn if an error occured

There's now a test that checks the error message when no pygmentize tool
can be found on the PATH.

modules/pygmentize.pl

@@ -52,9 +52,14 @@ sub DoPygmentize {
 
   RequestLockDir('pygmentize') or return '';
   WriteStringToFile("$TempDir/pygmentize", $contents);
-  my $output = `pygmentize $lexer -f html -O encoding=utf8 $args -- \Q$TempDir/pygmentize\E`;
+  my $output = `pygmentize $lexer -f html -O encoding=utf8 $args -- \Q$TempDir/pygmentize\E  2>&1`;
   ReleaseLockDir('pygmentize');
 
   utf8::decode($output);
+
+  if ($?) {
+    $output = $q->p($q->strong($output)) # "sh: pygmentize: command not found"
+        . $q->pre($contents);
+  }
   return $output;
 }

t/pygmentize.t

@@ -0,0 +1,25 @@
+# Copyright (C) 2015  Alex Schroeder <alex@gnu.org>
+#
+# This program is free software; you can redistribute it and/or modify it under
+# the terms of the GNU General Public License as published by the Free Software
+# Foundation; either version 3 of the License, or (at your option) any later
+# version.
+#
+# This program is distributed in the hope that it will be useful, but WITHOUT
+# ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
+# FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License along with
+# this program. If not, see <http://www.gnu.org/licenses/>.
+
+require 't/test.pl';
+package OddMuse;
+use Test::More tests => 2;
+
+add_module('pygmentize.pl');
+
+$ENV{PATH} = '.'; # pygmentize is not installed in the current directory
+$page = apply_rules(newlines('{{{\ntest\n}}}\n'));
+test_page($page,
+           'sh: pygmentize: command not found',
+           '<pre>test</pre>');