Feature Request: error-message handler to make you aware you don't have an S3 bucket hooked up in Heroku

[markyv18]

Problem
Sometimes you forget to plug in all the config vars ahead of time, or perhaps you don't know you need to. Then when you as an admin or owner are in the campaign summary and you click 'export data' and nothing happens you are confused! And there's no explanation given as to why it isn't exporting the data!

Solution
The solution of course is to enter your AWS_ACCESS_KEY_ID, AWS_S3_BUCKET_NAME, and AWS_SECRET_ACCESS_KEY in heroku but in order to know that is what you need to do we could use a little friendly nudge when we're in the app wondering what next to do! If upon clicking "export data", and with no AWS bucket hooked up, could we get an error message stating that these vars need to be filled in? Thanks!

Context
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