RFR: Report oops and fwdptrs verification failures fully

[Aleksey Shipilev]

In our current Verifier, we have to check obj and fwdptr before starting deeper
verification. The downside for that is, if obj or fwdptr are broken, we would
never get a full verification report, but rather failure on guarantee(...)-s.
Which is inconvenient, because it would not mention the phase where it failed,
the location the object was referenced from, etc.

The better way to handle this would be carefully verify in steps, then knowing
during the failure print what we can deem safe, and what we cannot:
  http://cr.openjdk.java.net/~shade/shenandoah/verifier-safelevels/webrev.01/

For example, new hs_err for a code root pointing to a dead region would look
like this:

http://cr.openjdk.java.net/~shade/shenandoah/verifier-safelevels/hs_err_pid24663.log

Testing: hotspot_gc_shenandoah, failing testcases

Thanks,
-Aleksey