Return clear error message when CIDR string is invalid in policy

[micahlee]

Currently, loading a policy with an invalid CIDR value value fails with a postgres exception and a 500 Internal Server Error response. Instead this should return a structured policy validation error like we have for other validations:

{
  "error": {
    "code": "validation_failed",
    "message": "policy_text Error at line 2, column 39 in  ...ge\":\"Error at line 2, column 39 in  : No such attribute 'restricted_to' on type HostFactory\""
  }
}

[micahlee]

This is implemented in #1762