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generate_test_data.sh uses `hdiutil create` and `hdiutil convert`. The former
can overwrite an existing output file, but the latter cannot. This led to the
following error:

  hdiutil: convert failed - File exists

But this message was getting swallowed because the script 1) did not do `set -e`
to stop on errors, and 2) redirect all output to /dev/null. This also fixes both
of those issues.

BUG=696529
R=thakis@chromium.org

Review-Url: https://codereview.chromium.org/2748453002
Cr-Commit-Position: refs/heads/master@{#456141}