templater: replace Py3-only exception types by super-types available in Py2
As noted by @indygreg, test-check-pyflakes.t started failing on Py2
after my recent D8894, because that introduced catching of the
Py3-only types ModuleNotFoundError and FileNotFoundError. Let's
switch to less precise types that are also available in Py2.
Differential Revision: https://phab.mercurial-scm.org/D8902