Converts errno.h error codes into GIO error codes. The fallback
IOErrorEnum::Failed is returned for error codes not currently
handled (but note that future GLib releases may return a more
specific value instead).
errno is global and may be modified by intermediate function
calls, you should save its value as soon as the call which sets it
Error number as defined in errno.h.
IOErrorEnum value for the given errno.h error number.