Log short errors for known errors

[dlau]

For errors with known messages, just log out the message. Don't give an Error to winston so it won't print a stack trace. If we have a message, it should be clear where the error came from.

error: [LaunchDarkly] Error: Unexpected error: "message"
    at /.../node_modules/ldclient-node/index.js:73:19
    at EventSource.es.onerror (/.../node_modules/ldclient-node/streaming.js:19:7)
    at emitOne (events.js:96:13)
    at EventSource.emit (events.js:188:7)
    at _emit (/.../node_modules/ldclient-node/eventsource.js:188:17)
    at EventSource.foo (/.../node_modules/ldclient-node/eventsource.js:40:5)
    at Object.StreamProcessor.processor.start (/.../node_modules/ldclient-node/streaming.js:86:8)
    at Object.new_client [as init] (/.../node_modules/ldclient-node/index.js:64:22)
    at Object.<anonymous> (/.../fake-node-client/index.js:3:31)
    at Module._compile (module.js:570:32)

will now look like this:

error: [LaunchDarkly] Error: "message"