oscap-docker: Print error message when target is not RHEL
oscap-docker: Print error message when target is not RHEL

ybznek/openscap,ybznek/openscap,OpenSCAP/openscap,Hexadorsimal/openscap,jan-cerny/openscap,OpenSCAP/openscap,Hexadorsimal/openscap,Hexadorsimal/openscap,ybznek/openscap,redhatrises/openscap,mpreisler/openscap,jan-cerny/openscap,jan-cerny/openscap,Hexadorsimal/openscap,redhatrises/openscap,redhatrises/openscap,jan-cerny/openscap,mpreisler/openscap,mpreisler/openscap,OpenSCAP/openscap,mpreisler/openscap,OpenSCAP/openscap,ybznek/openscap,Hexadorsimal/openscap,OpenSCAP/openscap,redhatrises/openscap,ybznek/openscap,OpenSCAP/openscap,jan-cerny/openscap,mpreisler/openscap,Hexadorsimal/openscap,jan-cerny/openscap,redhatrises/openscap,redhatrises/openscap,ybznek/openscap,mpreisler/openscap