Initial implementation of #1892
- Modifies the backend to return `duplicate_org_name` or
`duplicate_org_slug` as appropriate on a pymongo `DuplicateKeyError`
- Updates frontend to handle `duplicate_org_name`, `duplicate_org_slug`,
and `invalid_slug` error details
- Update errors to be more consistent, also return `duplicate_org_subscription.subId` for duplicate subscription instead of the more generic `already_exists`
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Co-authored-by: Ilya Kreymer <ikreymer@gmail.com>
