Propagate errors from PostConnect to the filter graph.

This fixes an issue when DXVA2 Native is refused during PostConnect,
and the software decoder is unavailable (wmv9 dmo not present)
Destruct the decoder object when init failed.

This avoids a half-initialized decoder lingering around and possibly
causing issues.
Check that the decoder object exists before dispatching commands to it.

This fixes an occasional crash when the decode thread enters an
unpredicted error state.