Improve error handling

[tkurki]

Dec  8 11:11:17 Signal_K sea_vu_play[352]: Unable to convert:Cannot read property 'Temperature Instance' of undefined:{"timestamp":"2017-12-08T17:11:14.824Z","prio":5,"src":114,"dst":255,"pgn":130312,"description":"Temperature"}
Dec  8 11:11:17 Signal_K sea_vu_play[352]: TypeError: Cannot read property 'values' of undefined
Dec  8 11:11:17 Signal_K sea_vu_play[352]:     at ToSignalK._transform (/home/pi/signalk-server-node/providers/n2k-signalk.js:33:34)
Dec  8 11:11:17 Signal_K sea_vu_play[352]:     at ToSignalK.Transform._read (_stream_transform.js:167:10)
Dec  8 11:11:17 Signal_K sea_vu_play[352]:     at ToSignalK.Transform._write (_stream_transform.js:155:12)
Dec  8 11:11:17 Signal_K sea_vu_play[352]:     at doWrite (_stream_writable.js:333:12)
Dec  8 11:11:17 Signal_K sea_vu_play[352]:     at writeOrBuffer (_stream_writable.js:319:5)
Dec  8 11:11:17 Signal_K sea_vu_play[352]:     at ToSignalK.Writable.write (_stream_writable.js:245:11)
Dec  8 11:11:17 Signal_K sea_vu_play[352]:     at N2KAnalyzer.ondata (_stream_readable.js:555:20)
Dec  8 11:11:17 Signal_K sea_vu_play[352]:     at emitOne (events.js:96:13)
Dec  8 11:11:17 Signal_K sea_vu_play[352]:     at N2KAnalyzer.emit (events.js:188:7)
Dec  8 11:11:17 Signal_K sea_vu_play[352]:     at readableAddChunk (_stream_readable.js:176:18)
Dec  8 11:11:17 Signal_K sea_vu_play[352]:     at N2KAnalyzer.Readable.push (_stream_readable.js:134:10)
Dec  8 11:11:17 Signal_K sea_vu_play[352]:     at N2KAnalyzer.Transform.push (_stream_transform.js:128:32)
Dec  8 11:11:17 Signal_K sea_vu_play[352]:     at Interface.<anonymous> (/home/pi/signalk-server-node/providers/n2kAnalyzer.js:50:12)
Dec  8 11:11:17 Signal_K sea_vu_play[352]:     at emitOne (events.js:96:13)
Dec  8 11:11:17 Signal_K sea_vu_play[352]:     at Interface.emit (events.js:188:7)
Dec  8 11:11:17 Signal_K sea_vu_play[352]:     at Interface._onLine (readline.js:232:10)
Dec  8 11:11:17 Signal_K sea_vu_play[352]:     at Interface.<anonymous> (readline.js:375:12)

A better error message, without the stack trace, is in order.