RawSocket: add meaningful handshake error codes

[danmarcab]

The router returns integer error codes when the handshake is not successful

0: illegal (must not be used)
1: serializer unsupported
2: maximum message length unacceptable
3: use of reserved bits (unsupported feature)
4: maximum connection count reached
#5 - 15: reserved for future errors

At the moment we return {:error, error_code}. We should return a better error message