Improve error message on mismatched scatter parameter.
Improve error message on mismatched scatter parameter.

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