fix error condition check in digitalWrite

This fixes following the error when I call: core.tinker.digitalWrite('D7', 'HIGH', callback);

TypeError: Cannot read property 'message' of null
    at localCb (/home/tonic/node_modules/spark-tinker/lib/tinker.js:48:12)
    at Spark.emitAndCallback (/home/tonic/node_modules/spark/lib/spark.js:130:42)
    at null.<anonymous> (/home/tonic/node_modules/spark/lib/spark.js:148:10)
    at Request.self.callback (/home/tonic/node_modules/spark/node_modules/request/request.js:373:22)
    at Request.EventEmitter.emit (events.js:98:17)
    at Request.<anonymous> (/home/tonic/node_modules/spark/node_modules/request/request.js:1318:14)
    at Request.EventEmitter.emit (events.js:117:20)
    at IncomingMessage.<anonymous> (/home/tonic/node_modules/spark/node_modules/request/request.js:1266:12)
    at IncomingMessage.EventEmitter.emit (events.js:117:20)
    at _stream_readable.js:920:16