Show validation errors of pre-composed documents

[eduardoboucas]

When the insertion of a pre-composed document generates an error, the main request fails with a 400, but shows a non-descriptive "Validation failed" message. We need to make sure that the original validation errors bubble up to the main request.

The format of the error is a question, as we need to indicate the document that failed, the field it belongs to as well as the (nested) fields that caused the problem, if applicable.