Issue #317: Fix oscap-docker traceback
Issue #317: Fix oscap-docker traceback

Oscap-docker gave a traceback when providing a wrong image ID
or wrong container ID.
This commit fixes the traceback by catching the exception
and writes an error message on stderr.

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