ext/flac/gstflacenc.*: Save the flow return from the last gst_pad_push() and make sure we pass the right flow return ...

Original commit message from CVS:
* ext/flac/gstflacenc.c:
* ext/flac/gstflacenc.h:
Save the flow return from the last gst_pad_push() and
make sure we pass the right flow return value upstream
in the case of failure; minor clean-ups.