Cumulus 331 download s3 file non-existent key

[sethvincent]

Summary: Make sure the downloadS3File function catches readstream errors by using pump. Add tests for downloadS3File.

Addresses CUMULUS-331

Changes

• use pump module to catch errors in both the read and write streams
• add test for case when key does not exist in bucket
• add test for case when the key does exist
• improve jsdocs and variable naming for downloadS3File

Test Plan

Things that should succeed before merging.

• Unit tests
• Adhoc testing
• Update CHANGELOG
• Run ./bin/eslint-ratchet and verify that eslint errors have not increased
  • Commit .eslint-ratchet-high-water-mark if the score has improved

[laurenfrederick]

You might need to get latest and do an npm install. I think Marc updated the eslint version which caused the spike in errors.

[kkelly51]

LGTM