Add data help

Improved the text of the error displayed when something goes wrong on the Add Data panel.

![image](https://cloud.githubusercontent.com/assets/924374/3756748/5fbf5d72-1831-11e4-90f5-9fa70b707e7f.png)

![image](https://cloud.githubusercontent.com/assets/924374/3756756/7bffe894-1831-11e4-8731-2ed2248a0039.png)

Fixes #228.