Fix handling of discovery errors

[SpComb]

See #22

• Change discovery:Discovery.receiver to just log Packet.unpack errors caused by invalid response packets, instead of shutting down the entire discovery with an error

• Improve the discovery:Packet.unpack error reporting

  2017/12/03 16:46:43 Discovery.receiver: received invalid packet from 127.0.0.1:40961: Invalid field: []byte{0x66, 0x6f, 0x6f}
  

• Add tests for the discovery package