Fix 500 errors when missing theme CSS is requested

[brian-kephart]

I've been getting a few 500 errors when my CDN requests expired theme asset files. This is caused by CamaleonController#render_error looking for a template to render instead of just returning a 404 header. This pull request preserves existing behavior for HTML and returns a 404 status for other request types.

This is not the only possible fix. If you'd prefer to only alter behavior for CSS, it can be done like this instead:

respond_to do |format|
  format.css { head status }
  format.any { render "camaleon_cms/#{status}", :status => status }
end

But I thought better to make the change for any request type that is not rendering an HTML page.

Yet another option would be to create a blank CSS file in the directory so there's something to render.