providing a string for an input with type File needs a better error message

`minimal.cwl`
``` cwl
#!/usr/bin/env cwl-runner
cwlVersion: v1.0
class: CommandLineTool
baseCommand: echo
inputs:
required_file: File
outputs:
path: stdout
```

`inputs.yml`:
``` yaml
required_file: "not_a_file"
```

```
$ cwltool minimal.cwl inputs.yaml
/home/mcrusoe/src/toil/venv/bin/cwltool 1.0.20180403145700
Resolved 'minimal.cwl' to 'file:///home/mcrusoe/common-workflow-language/v1.0/minimal.cwl'
Workflow error, try again with --debug for more information:
Invalid job input record:
null.yaml:1:1: the `required_file` field is not valid because
is not a dict
```