codecs: {h264,h265,mpeg2}decoder: Use GstFlowReturn everywhere

boolean return value is not sufficient for representing the reason of error in most cases. For instance, any errors around new_sequence() would mean negotiation error, not just ERROR. And some subclasses will allocate buffer/memory/surface on new_picture() but it could be failed because of expected error, likely flushing

Closes: gst-plugins-bad#1578 (closed)

Moved from gst-plugins-bad!2541 (closed)