MSVC compiler dll export warning 4275 with format_error

[ToddWilsonBertec]

Previously, the macro FMT_CLASS_API was suppressing the 4275 warning message when format_error was declared. Since this has been removed, the format_error decl vomits a bunch of warnings when compiled with MSVC.

To fix, change:

class FMT_API format_error : public std::runtime_error {

to

class FMT_API format_error : FMT_MSC_WARNING( suppress : 4275 ) public std::runtime_error {

[vitaut]

I don't think format_error even needs to be exported considering that it doesn't have any methods of its own. We could probably drop FMT_API from it.

[vitaut]

Should be worked around in e0fc0e8.