Clarify HAVE_PYTHON_APT/install_python_apt handling in apt_repository
Clarify HAVE_PYTHON_APT/install_python_apt handling in apt_repository

1. Don't test check_mode in both the caller and in the callee.
2. Don't test HAVE_PYTHON_APT inside an if that tests HAVE_PYTHON_APT
3. Don't be irritatingly vague about why the module fails ("You may be
   seeing this because…").

Note that if «apt-get -y install python-apt» succeeds with rc==0, but
for some reason python_apt is not usable afterwards, this will break
because the imports in install_python_apt aren't wrapped inside a
try/except.

In other words, we assume that install_python_apt either succeeds or
fails with a traceback. This commit doesn't affect that behaviour.

thaim/ansible,thaim/ansible