templater: add exception-raising version of open_template() I'm about to add another caller of `open_template()` (in the template loader). That caller will want to get exceptions instead of `(None, None)` if the template doesn't exist. This patch therefore changes `open_template()` to raise exceptions and adds a new `try_open_template()` that returns the `(None, None)` value. Differential Revision: https://phab.mercurial-scm.org/D8905