Expose Item validation errors via HTTP 500

[NfNitLoop]

I'm writing a new rss->feoblog client for Deno. I was getting a 500 "Server Error" from the server. That's not super helpful. Should expose what failed validation.

I saw this on my actix web console:

[2021-07-04T02:05:33Z ERROR actix_http::response] Internal Server Error: Error { inner: ValidationError { message: "Timestamp is required" } }

Make sure that gets returned via HTTP.

[NfNitLoop]

This would be an excellent thing to write an automated test for. Might be fun to set up an automated testing framework w/ this case.