When a workflow cannot be loaded because `--enable-ext` was not provided, the error message does not explain this

[adamnovak]

As noted in DataBiosphere/toil#5528, when a CWL workflow depends on an extension as a requirement, and --enable-ext is not provided, you get a message like this:

ERROR Tool definition failed validation:
mpich-mpirun-mpi-requirement.cwl:52:1: checking field 'requirements'
mpich-mpirun-mpi-requirement.cwl:54:3:   checking item
                                           Field 'class' contains undefined reference to
                                           'http://commonwl.org/cwltool#MPIRequirement'

This is the same error message as if the requirement were somehow misspelled. It does not indicate that the workflow might become understandable to cwltool if --enable-ext were provided, or even that the problem is that an unknown extension is required by the workflow, a situation we anticipate (rather than the workflow being corrupted or malformed in an unexpected way). It also talks about a class field, which doesn't actually appear in the document, since I am using the format where requirements is a YAML dict and the requirement name is the key.

This error message should be improved so that, when the user runs a workflow that needs --enable-ext without that flag, the output text suggests trying it with --enable-ext.

It should also be improved to explain that the problem is that cwltool does not (as currently configured) support the blah extension, as used in file blah.cwl.

[adamnovak]

Ideally this would be done in a way that Toil can pick up for free when it runs cwltool's parser.