Improve the error reporting for invalid option values. Now the

'validation_error' exception allows to set the name of the option, and the
what() method uses the name to generate better message.


[SVN r25141]

libs/program_options

@@ -1 +1 @@
-Subproject commit 72fda8877a01cad1cc1dcfc36226b124a3de3532
+Subproject commit ca1531d210b28957ee9079a980e76be323589acf