Catch responseText errors when responseType !== 'text' || ''

d3.xhr tests `request.responseText` which throws an uncaught "InvalidStateError" exception if responseType is not the empty string or "text". ([spec](http://xhr.spec.whatwg.org/#the-responsetext-attribute))

See [this example](http://bl.ocks.org/gmaclennan/520b8ccf857a36db5cf3) (open the console to see uncaught error).

I think this PR should catch the error.