Add validation for $priority

Valid range changed to 401-599:

vhost SSL header has priority 700. If $priority is set to 400 (which
resolves to 400+300=700 for SSL locations), then it would conflict with
the priority of the header. It must be 401 or higher to avoid this.

Top end of range is limited to 599 to reflect documentation however it
could be increased to 698 and still provide expected behavior.