add a __str__ implementation to APIException

Add a __str__ implementation to rest_framework.exceptions.APIException. This helps for logging raised exceptions. Thanks.
Merge pull request #1501 from jacobg/patch-1

add a __str__ implementation to APIException
replace page with page_size to avoide confusion
Merge pull request #1510 from uploadcare/docs-page-vs-page-limit

replace page with page_size to avoide confusion