show proper error message when viewing Customizer in IE8

[cvrebert]

Closes #13090.
Modulo the <strong> tag I've since added, here's what this looks like:

Note that this only shows in IE<9.

@mdo Sound acceptable?

[mdo]

Can we not use a huge alert and instead keep it a sane, default size, and then add some IE-specific styles to simply hide the container and sidebar? Something like:

<!--[if lt IE 9]>
<style>
  .bs-docs-section,
  .bs-docs-nav {
    display: none;
  }
</style>
<![endif]-->

[cvrebert]

Yeah, that could work.

[cvrebert]

@mdo Okay, done.

[mdo]

Fantabulous. Merge at your discretion <3.