Update Http return code for db insert/update failures

[qian-squareup]

What and Why

Update return code for db insert/update errors from the current 503 to 400.

Testing

{"writes":[],"error":{"message":"failed to insert entity (db err: write exception: write errors: [Document failed validation: {\"failingDocumentId\": {\"$oid\":\"684373427262043d854bfdf4\"},\"details\": {\"operatorName\": \"$jsonSchema\",\"schemaRulesNotSatisfied\": [{\"operatorName\": \"required\",\"specifiedAs\": {\"required\": [\"app\",\"aws_arn\",\"aws_account_id\",\"aws_region\",\"cache_node_id\",\"replication_group_aws_arn\"]},\"missingProperties\": [\"aws_arn\",\"aws_account_id\",\"aws_region\",\"cache_node_id\",\"replication_group_aws_arn\"]}]}}])","type":"db-insert-failed","entityId":"","httpStatus":400}}