Towards 5027 -- add options hash to lookupvar as with setvar

This patch adds an options hash to lookupvar analogous to the one taken by
setvar and uses it to pass in source location for error reporting. It also
fixes the mechanism used by setvar (file was not being passed correctly), adds
line and file information to errors in templates, and extends/corrects tests.

As presently written it does not gather userful line numbers from inline
templates and there are no tests for the template line number generation.