Improves error objects

[n-johnson]

Fixes #2

This updates the response handling code to ensure the response body is parsed as json before trying to use the request body in the error handling code.

Error before (currently on master):

{ WebflowError
    at /Users/nathan/webflow/node-webflow-api/dist/Webflow.js:95:37
    at process._tickCallback (internal/process/next_tick.js:103:7)
  _meta: { rateLimit: { limit: 60, remaining: 58 } },
  code: undefined,
  msg: undefined }

And after (as it is in this PR):

{ WebflowError: ValidationError: Validation Failure
    at /Users/nathan/webflow/node-webflow-api/dist/Webflow.js:70:17
    at process._tickCallback (internal/process/next_tick.js:103:7)
  code: 400,
  msg: 'Validation Failure',
  _meta: { rateLimit: { limit: 60, remaining: 58 } },
  problems: [ 'Field \'l\': Field not described in schema' ] }

[jacobwgillespie]

LGTM!