Pass through status in plc_tag_status

Previously only returned PLCTAG_ERR_NOT_IMPLEMENTED
in plc_tag_status if the tag->vtable or tag->vtable->status
were false. This resulted in many different errors being
hidden behind one error code.

Added code where when the above condition occurs, the tag->status
will be passed through if it is non-zero.
Merge pull request #4 from ericorange/plc_tag_status

Pass through status in plc_tag_status