Reland: Require TARGET_RELEASE for builds

Relanding because we've fixed a number of the broken build
setups this change exposed.  We will push to fix forward broken
build setups instead of reverting this change again.


Rather than use an unsupported flag setting that the user likely
doesn't even realize is being used, we immediately stop the build.

This error message is more verbose, mentioning 'lunch', because
it's anticipated a lot more users will hit this issue when first
switching to trunk stable, and more details will hopefully help
them out.

We have some complication in that some internal commands set
TARGET_RELEASE to an empty string.  We put in logic to allow
that path.

Since $(error) immediately stops the build, we also get rid of
some 'else' logic and indentation, to hopefully offset some of
the complication we've added.

Bug: 307946156
Change-Id: I0fa4a1c876e607401f4c7f945b9971cfb8db71a0
Test: 'lunch' (still) works; A build attempt without `TARGET_RELEASE` set (now) fails