Better error message when trying to render a template that has been filtered out

[imalsogreg]

Running gHeistSingleServe on a template that has been filtered out by hcCompiledTemplateFilter (especially templates beginning with '_') reports ERROR: Template "_foopage" not found. This can be confusing because the documentation suggests that underscored templates can be served by heistServeSingle, and the filtering out of underscored templates is a default in the Heist snaplet initializer.

It would be nice to have a better error message in these cases. cc @kaol