Print a user-friendly message when `lunch` cannot use the default product.

In some environments (e.g. with the master-art branch), the default
product (aosp_arm-eng) cannot be built, and thus prevents `lunch` from
printing the lunch menu (as it cannot initialize the build system in
order to fetch the common lunch choices). When this happens, show an
explicit error message from `lunch` instead of a Soong UI error.

Test: Check that this command line displays the expected error message:
        unset TARGET_PRODUCT && . build/envsetup.sh && lunch
Bug: 152762648
Change-Id: I40c66bca5b075d88dbc8364b36b2db713e6ad93e