Improve data validation experience

[gnavvy]

Summary

Several users have reported (#103, #108) that no results were shown after uploading CSV files, possibly due to format/column mismatch.

Expected Behavior

Users should be able to get detailed warning/error messages if data validation has failed.

Current Behavior

Nothing being rendered and no actionable message is surfaced for the cause of the format mismatch.

Possible Solution

1. As an MVP, output (e.g. console.log) if there are any expected columns missing.
2. A wizard to guide the data column mapping and validation.

[Firenze11]

These are great ideas. We have two items in the roadmap that correspond to these two suggestions:

[P0] Improve error messages for input data
[P1] Allow users to monitor and change feature types