Missing error message when using empty file

## Issue
When running the cwltool with an empty CWL or YML file, the error message is empty.


## Full Traceback
```pytb
INFO /rnaseq-cwl-training-exercises/env/bin/cwltool 3.1.20220224085855
INFO Resolved 'rna_seq_workflow.cwl' to 'file:/rnaseq-cwl-training-exercises/rna_seq_workflow.cwl'
ERROR I'm sorry, I couldn't load this CWL file.
The error was:
Traceback (most recent call last):
File "/rnaseq-cwl-training-exercises/env/lib/python3.8/site-packages/cwltool/main.py", line 1109, in main
loadingContext, workflowobj, uri = fetch_document(uri, loadingContext)
File "/rnaseq-cwl-training-exercises/env/lib/python3.8/site-packages/cwltool/load_tool.py", line 133, in fetch_document
loadingContext.loader.fetch(fileuri, content_types=CWL_CONTENT_TYPES),
File "schema_salad/ref_resolver.py", line 990, in fetch
StopIteration
```

## Your Environment
* cwltool version: 3.1.20220224085855