GIO: Convert ENOTSOCK to G_IO_ERROR_INVALID_ARGUMENT
instead of using a generic G_IO_ERROR_FAILED error code. This is in line with what W32 part of the code is doing with WSAENOTSOCK.
This fix will break two tests in libsoup, which were written following the implementation and thus expect G_IO_ERROR_FAILED when attempting to do stuff with no-longer-valid socket descriptors.
Closes #1728 (closed)