@Override
  public void addErrors(Collection<String> errors) {
    this.errorSet.addAll(errors);
  }
An extension version is like a stack version but it contains custom services.  Linking an extension
version to the current stack version allows the cluster to install the custom services contained in
the extension version.
ambari-server/src/main/java/org/apache/ambari/server/controller/ExtensionLinkResponse.java