TemplateHooks could easily leak data into the main context when

rendering, overriding variables provided by other contexts. While

TemplateHook tries to push/pop, subclasses can always override and

forget to do so. They can also potentially raise an exception, breaking

the rendering of the page.
To work around all this, we now push/pop in template_hook_point, to

prevent any leakage, and catch exceptions to help sandbox.

Unit tests pass.
First saw this issue when rbmotd's custom TemplateHook was writing directly

into the context, causing issues with the Power Pack config page. After this

change, it all worked properly.