When compiling {fmt} as a DLL, MSVC complains that we are exporting
classes that inherit from "std::runtime_error", which we are not
exporting.

In this case, it's not really a problem because that symbol is already
exported via the C++ stdlib. So we just add a pragma to silence the
warning.

buffered_file& operator=(buffered_file&& other) calls close which can
throw.