Additional logging during failure

In the process of trying to use the test client, I made a mistake between function types and it caused a failure:

```
| 2020/10/21 22:30:29 Validating for http...
| 2020/10/21 22:30:29 Framework server started.
| 2020/10/21 22:30:30 HTTP validation started...
| 2020/10/21 22:30:30 Framework server shut down. Wrote logs to serverlog_stdout.txt and serverlog_stderr.txt.
| 2020/10/21 22:30:30 Validation failure: failed to unmarshal json: json: cannot unmarshal object into Go value of type string
```

It would have been helpful to have more details about what caused the JSON to fail to be unmarshalled here. Perhaps a `--verbose` flag would include this?