Require TARGET_RELEASE for builds

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
Test: 'lunch' (still) works; A build attempt without `TARGET_RELEASE` set (now) fails
Change-Id: I2f667668c6688e501a3536981b7bae5fdbf962a0