Show import errors on frontend #731
Description
As someone importing data with a spreadsheet file to Vihko or Data Bank, I want to see clear error messages on the UI, so that I can fix errors.
Currently UI sometimes shows only generic errors, like "gatheringEvent". The UI should show more detailed errors that are provided by the validation API, e.g. "MA codes must be yourself or your friend", in case user tries to use non-friend AM codes as observers.