Review how errors can occur for users and provide graceful fails / UX messaging

[MichaelHanksSF]

Two day resource cap.

Can follow #111

[christina-moore]

Situations to consider:

• No files are available in storage
• Files available are not properly formatted/do not contain the necessary columns

[christina-moore]

Upon further investigation I've decided not to add extra error handling. Now that sentry is enabled (or is in PR to be enabled) critical errors (like files being missing) will be reported there and then presumably to a slack channel for us to see. We could give feedback to the user, but the entire site is unusable in these instances anyway so we would just be telling users to alert us. I think it would be better to ensure the freshdesk email is available, perhaps on the home page and then to let sentry report the errors.

Furthermore, files being absent from storage should never occur - files are only programmatically deleted when new ones are validated and uploaded. And the upload validator runs a prediction that checks for necessary columns so that should not happen either. These situations could occur through "extraordinary" circumstances where someone tampers directly with the files in the S3 bucket. Again, sentry error reporting should be a strong enough measure there.