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

mercurial/templater.py

@@ -1107,5 +1107,5 b' def open_template(name, templatepath=Non'
                 name,
                 resourceutil.open_resource(package_name, name_parts[-1]),
             )
-        except (ModuleNotFoundError, FileNotFoundError):
+        except (ImportError, OSError):
             return None, None