add processing_errors to SIS imports API docs

SIS imports have a processing_errors key that is not included
in the current API docs.

Test Plan:
- visit /doc/api/sis_imports.html
- the SisImport object should include processing_errors

Refs SIS-432

Change-Id: Ifd930a513f3f802a0a6354e84716562aab2f0d35
Reviewed-on: https://gerrit.instructure.com/39704
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Ken Romney <kromney@instructure.com>
Product-Review: Duane Johnson <duane@instructure.com>
QA-Review: Jeremy Putnam <jeremyp@instructure.com>