Separate render and getHtml

Currently the method render always returns
a html string that can be a string that
represents the correct result or a rendered
error message.
This change splits rendering and
fetching of the HTML output.
This simplifies the logic of the rendering
and caching mechanism.
Now the render method returns a boolean that
indicates if the rendering was sucessful or
not.

Step 2/2

Warning: Errors in this change might affect
caching logic and squid caches.

Change-Id: I00502b84212ed70cdf63bd69916a35afbd6fdbc1