show proper error message when viewing Customizer in IE8

Closes #13090.
Modulo the `<strong>` tag I've since added, here's what this looks like:
![ie8](https://cloud.githubusercontent.com/assets/419884/3011003/02634c02-df23-11e3-9e82-d3bbd83b185f.png)
Note that this only shows in IE<9.

@mdo Sound acceptable?