Added a MaterialisingResponse

It executes the deferred body immediately and stores the results in memory to be sent on later.

Wired it into the ViewProcessor to catch template execution errors if error traces are on - added a demo for it to the main asp.net project.

Missing tests, so not ready to pull in yet, just putting this here to see if it fits the bill. The error message could be better, but not if there's anything we can do about that - the template has been and gone at that point.