template_depends: throw nice error message when template cannot be found

plovs reported a crash when templates were not installed properly,
with a non-useful error about the template object not being defined.
I've audited all uses of template_depends(), and template(), and it makes
sense for them to throw an error if the template cannot be found. All code
with a user-supplied template catches errors already, to handle template
parse failures.

It did not make sense for template_file to throw errors, as some code uses
it to probe if a template file is available.