Dataset schema validation: add data property to API error object

[katacek]

When dataset schema validation fails, js client returns info about it as a part of the 400 error, it would be great if python client can do it as well.

When added, this part of docs can be updated as well (/dataset_schema/validation.md).

Docs PR for dataset schema validation here
How it was added in js here

[Pijukatel]

Added data to error: #314
Document update: apify/apify-docs#1356