Rename exception ctor argument to "what_arg"
Rename exception ctor argument to "what_arg"

This avoids a -Wshadow from g++ 4.8, which is still supported and uses
the same name as https://en.cppreference.com/w/cpp/error/runtime_error

No real changes.

vslavik/xmlwrapp,vslavik/xmlwrapp,vslavik/xmlwrapp