[KeyVault] - Use error object for LRO errors (Azure#15500)

## What

- Use `error.code` to check if there's an error (not just the presence of error)
- Unpend the Admin LRO tests

## Why

- Consistency with other languages
- We used to require using StatusDetails or status because the AAD errors were not populated in the error object, now that this is fixed we can use the error object.
- Even on a successful response `error` will still get sent over the wire, so we need to check the presence of `code` as well. This service-side issue will be fixed, but for now all languages will guard against this.

Fixes Azure#15448

Author: maorleger