Returning exception to callback method receiving invalid json

[manupsunny]

As there are no/limited error handling mechanisms available for the end user now, returning exception to the callback method on getting an invalid json back from Google service.

[EirikBirkeland]

manupsunny Please check out this one too which I did without first seeing your change:
#131

My version returns a status code such as 400 and 404 as soon as the xhr request fails.

I think our versions can be easily combined, since it may still be possible for the xhr request to succeed while the user still experiencing the same error later on. I will look into this later.

Also I think it would be nice to have standard messages for each HTTP status code based on what's usually the problem -- for example, an unpublished spreadsheet should result in a 403 error, while 404 is obviously an invalid URL, and 400 is some kind of less specific problem with the xhr request sent to the server (like header corruption I believe.

[EirikBirkeland]

And further in regard to HTTP error codes, I think I would actually prefer to customize these myself, but perhaps we could return a detailed error object, like this:

{
   code: 403,
   message: "403 Forbidden – Your request to view the public version of the spreadsheet was rejected by Google's servers. Please ensure that you have made your spreadsheet public."
}

Then the user would just deal with the first err parameter just like we both had in mind:

const mySheet = Tabletop.init(myUrl)

mySheet.fetch(err, res) {
  if (err) handleError(err)
  // do stuff with result here if no error
}