Update Errors page

[lesavage]

Updating the errors listed to remove redundant entries and to add new entries such as:

• error.throttled_by_external_service: this is returned when User Management API receives a 429 from another external service which is out with its control.
• error.usergroup.exceeds_maximum_member_count: this error can be returned for unusually large groups. From experience we know that provision or viewing these large groups can be problematic.

This PR also introduces a support page to help User Management API customers to find further guidance.