check for errors before trying to parse response

[tomwayson]

resolves #110

[coveralls]

Coverage remained the same at 100.0% when pulling 14f5668 on fix/response-ok into 2a30d22 on master.

[patrickarlt]

@tomwayson should this be an ArcGISRequestError? Mainly so users can avoid situations like having to check for Error vs ArcGISRequestError:

request(/* ... */).catch(error => {
  if(error instanceof Error) {
    // ok I have an error so I only have error.message...
  }

  if(error instanceof ArcGISRequestError) {
    // ok I have an ArcGISRequestError so I can access lots of other stuff...
  }
})

It might be easier if users can do this:

request(/* ... */).catch(error => {
  // error is always a ArcGISRequestError
})

This could be something like:

return Promise.reject(new ArcGISRequestError(statusText, `HTTP ${status}`));

which woudl result in error messages like HTTP 404: Not Found

[tomwayson]

I'll give that a go later today