[gtkmm-documentation] docs/tutorial/insert_example_code.pl: Add an error message

[Kjell Ahlstedt <kjellahl src gnome org>]

commit fbe0b0b33028ea52f9a5cb807efbf0e359d7c0dd
Author: Kjell Ahlstedt <kjellahlstedt gmail com>
Date:   Wed Oct 9 18:23:58 2019 +0200

    docs/tutorial/insert_example_code.pl: Add an error message
    
    Print a message to stderr when a file can't be opened.

 docs/tutorial/insert_example_code.pl | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)
---
diff --git a/docs/tutorial/insert_example_code.pl b/docs/tutorial/insert_example_code.pl
index fa3a84c..f65fdf9 100755
--- a/docs/tutorial/insert_example_code.pl
+++ b/docs/tutorial/insert_example_code.pl
@@ -74,7 +74,10 @@ sub process_source_file($$$)
   print "</para>\n";
   print "<programlisting>\n";
 
-  open(SOURCE_FILE, "$directory/$source_file");
+  if (!open(SOURCE_FILE, "$directory/$source_file"))
+  {
+    print STDERR "Can't open $directory/$source_file\n";
+  }
 
   while(<SOURCE_FILE>)
   {