feat: Batch requests with errors should include the errors in a response header

[w-le]

https://github.com/place-labs/staff-api/blob/f3f42b1b45ea1c7cfed2276b9a91a953e40d2084/src/controllers/events.cr#L45

Currently the number of errors is included in a response header.
It would greatly improve debugging experience if the error messages were also included in the response header or a new field in the response body.