Add more error checking when getting crash dumps on android.

For some reason we are failing to access the tombstones and crash dumps
sometimes (all the time, maybe?) and when that happens we crash further
along when we try to read nonexistent files.

TBR=jochen@chromium.org, peter@chromium.org
BUG=321489

Review URL: https://codereview.chromium.org/79393002

git-svn-id: svn://svn.chromium.org/blink/trunk@162397 bbb929c8-8fbe-4397-9dbb-9b2b20218538