Adding exception handling and reporting when trying to reconnect to the distributed server.

This is a partial fix for #1633 - 

When downloading data from KA, we currently have the central server POST back to a distributed URL.  Well, if the distributed server isn't web-accessible, this will fail.

**Previous behavior:**
The error wasn't caught, and this would leave the user hanging with a blank error page on the central server website.

**Current behavior:**
Now, the central server will catch the post-back error and post an error message back the distributed server user.

![screen shot 2014-03-05 at 2 22 06 pm](https://f.cloud.github.com/assets/4072455/2339449/e1c6b210-a4b5-11e3-80e2-fac99ca7d406.png)

**Testing:**
Tested this on adhoc.learningequality.org