Improve the heap verifier's failure report.

Improve the heap verifier's failure report.

Previously, when a reference verify failed we would print the location
of the bad reference and abort.  With this change, when a reference
fails to verify we try to print out more context.  First, we dump the
object which contains the bad references and print each reference that
failed verification.  Following that, we scan the roots and the heap
and print all of the objects which reference the bad object.