"CSV file has invalid content or its structure does not correspond to the mapping defined in the package." isn't very helpful.
More granularity in this error message would be very helpful. I just figured out that you probably don't like the NAs in non-string fields, but a more specific error message could have saved me time troubleshoorting. As this is my first experience with the platform, it occurred to me that more help troubleshooting might keep the less-technical from giving up in frustration. Catch the errors a bit earlier and pass more specific messages down to the catch which generates this message.