unhelpful exception when the wrong name is used in `scatter`

[mr-c]

https://cwl.discourse.group/t/scattering-doesnt-work/994

Should not print and exception, should print the file & line number plus a list of valid names.

Probably also should have been caught in the static checker.

[cwl-bot]

This issue has been mentioned on Common Workflow Language Discourse. There might be relevant details there:

https://cwl.discourse.group/t/scattering-doesnt-work/994/2