[JENKINS-32797] Break the catch clause contents of Jenkins.getTarget(… (#2652)

* [JENKINS-32797] Break the catch clause contents of Jenkins.getTarget() out into a separate, publicly accessible function.

This will allow plugins (particularly authentication plugins that override the normal authentication process) to determine if authentication is not required for a particular path by calling isPathUnprotected(restOfPath).

* Add @since TODO to comment

* Change name of function to something that is accurate and clear

isPathUnprotected is misleading, and the Javadoc was worse. isSubjectToMandatoryReadPermissionCheck is a much better name, and the return value is reversed to match the name,