Allocation summarize errors

[andrews05]

The Allocations implementation seems to be lacking standard handling of validation errors:

• There's no summarize_errors option for createCreditNoteAllocation().
• There's no status_attribute_string, has_validation_errors or validation_errors properties for the Allocation model.

(Granted, it only allows creating one allocation at a time anyway, but adding this support would allow using the same code flow for allocations as for other objects, and make it easier to distinguish validation errors from other 400 errors.)

[edit] The model is also missing the credit_note property.

[andrews05]

I've refiled this in the Xero-OpenAPI, closing here.