Refactor `csv_importer.py` and `csv_to_database.py` and `covidcast_row.py`

[dshemetov]

A number of refactors can be done here:

• csv_importer and csv_to_database are tightly coupled, but with low cohesion; would probably be best if merged together
• csv_importer validation could likely be handled by Pandas exceptions
• csv_importer can exit early when detecting a Pandas exception and save time
• csv_importer can probably share validation with covidcast_row