Give even better error messages on failure to launch ahat.

In particular, explicitly state that a heap dump "does not appear to
be a valid Java heap dump" on HprofFormatException.

Bug: 64318055
Test: m ahat-test
Test: ahat Android.mk
Test: ahat FileThatDoesNotExists.hprof
Test: ahat etc/L.hprof & ahat etc/O.hprof

Change-Id: I014b465e23e48941655bcbb3505cec8fd7965fc9