Tests: Improve BWC preconditions to error cleanly on major version difference.

This change makes running a BWC test on master against a 1.x version produce the following exception:

`java.lang.IllegalArgumentException: Backcompat elasticsearch version must be same major version as current. backcompat: 1.3.2, current: 2.0.0-SNAPSHOT`

It also fixes the @after cleanup to not throw an NPE if there was an error before the cluster was created (which is the case with this error). And finally it makes the backcompat version available (I needed this recently in the analysis bwc tests given a behavior change in lucene that needed to be worked around).
I reworked this as requested, but I'm still doing it where we grab the version in BWC.  The reason to do it there is the beforeTest is too late (the cluster is already created).  For this we should exit early.