(parsing) Some thrown exceptions's messages are missing important info about the offending options

[Denis N <dzinyss@…>]

The "unknown_option" exception has a message that looks like "unrecognized option".

This can be easily improved by modifying the line cmdline.cpp:427

to: boost::throw_exception(unknown_option(opt.string_key));

Similar issue with " multiple occurences" exception. The exception message does not specify which of the options occured multiple times. This can be fixed by adding the constructor parameter in "multiple_occurences" exception class, and passing it to the parent class.