Fix #2314 by ignoring deleted assets in zip download

[lindapaiste]

Fixes #2314

Changes:

• catch errors fetching files from AWS and include an empty 0KB file instead.
• Simplify addFileToZip by converting to async/await syntax.
• Use the return value of .folder() rather than manually concatenating paths.

I'm not sure if how I'm handing deleted files is the best way. It seemed like the best among the options that I considered. We can skip over missing files entirely. I also toyed with putting a text file (ie. image_png.txt) with the contents "File Not Found".

I have verified that this pull request:

• has no linting errors (npm run lint)
• has no test errors (npm run test)
• is from a uniquely-named feature branch and is up to date with the develop branch.
• is descriptively named and links to an issue number, i.e. Fixes #123

[PiyushChandra17]

👍

[raclim]

Thanks for your work on this!