Have `art/tools/buildbot-build --target` abort on empty TARGET_PRODUCT.

This is especially useful when the user has forgotten to run the
`lunch` command (or forgotten to define TARGET_PRODUCT manually), to
provide them with a meaningful error message instead of something
cryptic like:

  ninja: error: unknown target 'out/target/product/system/etc/public.libraries.txt'

Test: tools/buildbot-build.sh --target
Change-Id: Ia7e8819eb6902db8fd8f6ac24789f1b25c3a6f8c