Send an ofp_error_msg on flow add/delete error

Change the Voltha API to add the OF transaction ID (xid) to the request sent to the Core such that the Core can send an event back to ONOS using that same xid.

An OFPT_ERROR_MSG will be sent back to the ONOS. Structure of that message is below.

struct ofp_error_msg

{ struct ofp_header header; uint16_t type; uint16_t code; uint8_t data[0]; }

;

When an adapter fails to add flows then it is expected that it will return an error that can be mapped to a ofp_error_msg (both error type and code) and relayed back to the OF controller.