message: Update to the code example in GOLANG-HOWTO README
  This update fixes two minor issue with the example for "Storing and loading existing keysets" in the GOLANG-HOWTO README.
diff: @@ -552,7 +552,7 @@ func main() {
  }
  // Fetch the master key from a KMS.
  - gcpClient := gcpkms.NewClientWithCredentials(keyURI, credentialsPath)
  + gcpClient, err := gcpkms.NewClientWithCredentials(keyURI, credentialsPath)
  registry.RegisterKMSClient(gcpClient)
  @@ -561,7 +561,7 @@ func main() {
  log.Fatal(err)
  }
  - masterKey, err = aead.NewKMSEnvelopeAEAD(*aead.AES256GCMKeyTemplate(), backend)
  + masterKey := aead.NewKMSEnvelopeAEAD(*aead.AES256GCMKeyTemplate(), backend)
  if err != nil {
  log.Fatal(err)
  }
  