What should resolver.Run return when buffering.. error or content?

[eikenb]

The ResolveEvent that is returned during buffering doesn't have the content which would otherwise always be available with the caching template (PR #59) and the error is nil. This breaks with the other return semantics where it either returns a meaningful ResolveEvent with content OR returns a empty/default ResolveEvent and an error.

This can be addressed in either direction.

1. add a method to the template to let the resolver grab the cached content and return a meaningful ResolveEvent (w/ content and NoChange=true).
2. have it return an error when buffering with the empty/default ResolveEvent (like other errors)

I'm leaning towards option 2 but am not 100% yet.