[Bug libstdc++/70493] std::setlocale("") throws exception

[michi at triodia dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70493

--- Comment #1 from Michi Henning <michi at triodia dot com> ---
Ah, on reading http://pubs.opengroup.org/onlinepubs/9699919799/ section 8.2, it
appears that it's OK to throw in this case.

It would be nice to have a better diagnostic in the exception though. It could
show the facet that was found to be bad and the value that was objectionable
maybe?