Editor page not displaying errors for mismatched datasource types

Hi @dfeeney,

I believe I tracked down the issue we were having with asynchronous datasources not displaying their errors properly. To recreate the issue I did the following:
1. Go to the upload data screen
2. Select "From the Web" under "Simple Spreadsheets"
3. Enter the following URL: https://api.twitter.com/1/statuses/user_timeline.json?screen_name=ndiipp&count=20
4. Click upload

At this point the editor screen would hang. The problem is that Akara is failing and throwing a 500. Our front-end code was expecting Akara to set the response message to 'No Data'. When Akara throws a 500, no message is set. This commit considers this possibility and reloads the page to display an error message if it's encountered.