Convert OSError (for missing files) into validation error.
Convert OSError (for missing files) into validation error.

chapmanb/cwltool,jeremiahsavage/cwltool,dleehr/cwltool,dleehr/cwltool,dleehr/cwltool,SciDAP/cwltool,chapmanb/cwltool,SciDAP/cwltool,jeremiahsavage/cwltool,jeremiahsavage/cwltool,SciDAP/cwltool,common-workflow-language/cwltool,chapmanb/cwltool,jeremiahsavage/cwltool,SciDAP/cwltool,chapmanb/cwltool,common-workflow-language/cwltool,common-workflow-language/cwltool,dleehr/cwltool