Additional error handling in Benchmark script.
Additional error handling in Benchmark script.

This CL introduces detection of Chrome launch failures (run_in_snapshot fails), as well as adds Chrome shutdown to the teardown (previously, if an error was thrown from _DoIteration, the launched instance of Chrome would be left running).
Review URL: http://codereview.appspot.com/5078048

git-svn-id: db59699583a60be9a535cd09cdc9132301867226@430 15e8cca8-e42c-11de-a347-f34a4f72eb7d

google/syzygy,wangming28/syzygy,ericmckean/syzygy,sebmarchand/syzygy,wangming28/syzygy,supriyantomaftuh/syzygy,Eloston/syzygy,google/syzygy,ericmckean/syzygy,wangming28/syzygy,pombreda/syzygy,wangming28/syzygy,ericmckean/syzygy,sebmarchand/syzygy,Eloston/syzygy,supriyantomaftuh/syzygy,supriyantomaftuh/syzygy,sebmarchand/syzygy,google/syzygy,ericmckean/syzygy,supriyantomaftuh/syzygy,pombreda/syzygy,pombreda/syzygy,sebmarchand/syzygy,pombreda/syzygy,google/syzygy,sebmarchand/syzygy,pombreda/syzygy,ericmckean/syzygy