Resolve additional warnings

Resolve the following warnings:

• Annotation 'Author.class' is not retained for reflective access (in o.a.a.cli.PluginVerifier)
• o.a.a.cli.PluginVerifier uses unchecked or unsafe operations
• sun.security.validator.ValidatorException is internal proprietary API and may be removed in a future release (in o.a.a.servlet.WebResponder)