Add error handing for _loadData

[samchrisinger]

My concern is that if m.request fails or gets a non-200 response the developer interface exposes no way to catch and react to these errors.

In particular, if load data fails I want to destroy the grid and show an error message.

This is addressed to some extent here, https://github.com/caneruguz/treebeard/pull/71, but we should use the native error handler from m.request. (http://lhorie.github.io/mithril/mithril.request.html#using-variable-data-formats)