Expose error codes

[bennycode]

This PR solves /issues/58 - I am adding the code from the error payload to the error object, so that it can be checked.

Test code:

const Binance = require('./dist/index').default

const client = Binance({
  apiKey,
  apiSecret,
})

const order = {'symbol': 'TRXETH', 'side': 'SELL', 'type': 'LIMIT', 'quantity': '-1337', 'price': '1.00000000'}

client.order(order).catch(error => console.log('Error code', error.code))

[coveralls]

Coverage decreased (-23.8%) to 69.231% when pulling 796721c on bennyn:patch-3 into 6647d6d on HyperCubeProject:master.

[balthazar]

Perfect! I'm wondering if it would be interesting to expose some error types tight to these codes or would that not be helpful at all?

[bennycode]

That's a good idea! We can create enums for the error codes, so that checks can be done like this:

t.is(e.code, BinanceError.ILLEGAL_CHARS)

We can use the same identifiers as listed in the Error codes for Binance document, which are BinanceError.UNKNOWN, BinanceError.DISCONNECTED, BinanceError.UNAUTHORIZED, BinanceError.TOO_MANY_REQUESTS, BinanceError.UNEXPECTED_RESP, ...