Tweak error reporting on invalid workflow step.
Tweak error reporting on invalid workflow step.

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