Add a better error for when a user tries to provision past their quota

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#4605 introduced quotas and uses the InsufficientCapacity error added by @sunshowers in #4573. InsufficientCapacity generates a 5xx error but I believe we should both have a distinct error type and a 400 error code for when provisioning fails due to quotas. As mentioned in #4573, AWS has good documentation on their api errors. They have InsufficientCapacity as a 5xx server error related to their servers not having the resources to handle a request and 4xx client errors like VcpuLimitExceeded for when users try to over provision their allotted resources.