add revision key to failure message

[raycohen]

It would be useful to output the revision key that already exists in the "already uploaded" error message

ember-cli-deploy-s3-index/lib/s3.js

Line 78 in 1a76730

return RSVP.reject("REVISION ALREADY UPLOADED! (set `allowOverwrite: true` if you want to support overwriting revisions)");

I know it gets printed higher up in the verbose output, but its a little hard to find. I'd be willing to contribute a PR for this if there's interest.

[lukemelia]

Good idea!