Describes the cause of the error with structured details.

Example of an error when contacting the "pubsub.googleapis.com" API when it
is not enabled:

{ "reason": "API_DISABLED"
"domain": "googleapis.com"
"metadata": {
"resource": "projects/123",
"service": "pubsub.googleapis.com"
}
}

This response indicates that the pubsub.googleapis.com API is not enabled.

Example of an error that is returned when attempting to create a Spanner
instance in a region that is out of stock:

{ "reason": "STOCKOUT"
"domain": "spanner.googleapis.com",
"metadata": {
"availableRegions": "us-central1,us-east2"
}
}