android: make tombstones.py behave when /data/tombstones does not exist
android: make tombstones.py behave when /data/tombstones does not exist

When /data/tombstones does not exist on the device, tombstones.py
generates a loud traceback. Adding a check to make it behave.

BUG=552336
TEST=adb shell rm -rf /data/tombstones && build/android/tombstones.py

Review URL: https://codereview.chromium.org/1410123008

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#358336}

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