d/dns_managed_zone: Error out if zone is not found

[radeksimko]

Data source should almost never silently fail when the resource doesn't exist, because:

a. There's most likely a good reason user chose to use a data source instead of a resource. They don't manage the resource and expect it to be there and rely on it.
b. It leads to confusing errors, e.g.

data "google_dns_managed_zone" "gcp" {
  name = "bla-blah"
}

resource "google_dns_record_set" "www" {
  name = "www.${data.google_dns_managed_zone.gcp.dns_name}"
  type = "A"
  ttl  = 5

  managed_zone = "${data.google_dns_managed_zone.gcp.name}"
  rrdatas = ["10.10.0.2"]
}

Error applying plan:

1 error(s) occurred:

* google_dns_record_set.www: Resource 'data.google_dns_managed_zone.gcp' not found for variable 'data.google_dns_managed_zone.gcp.name'

Test results

TF_ACC=1 go test ./google -v -run=TestAccDataSourceDnsManagedZone_basic -timeout 120m
=== RUN   TestAccDataSourceDnsManagedZone_basic
--- PASS: TestAccDataSourceDnsManagedZone_basic (5.14s)
PASS
ok  	github.com/terraform-providers/terraform-provider-google/google	5.169s

[ghost]

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