There's currently no way for a OAuth2 plugin to redirect to the "unauthorized" page.
This can happen for instance when a user restrict the scope : he'll be authenticated but the plugin won't be able to get his information.
We should provide a org.sonar.api.server.authentication.UnauthorizedException for plugin.
This exception must NOT generate a stacktrace.
When this exception is generated, we'll display the message to the user.