Better error message on bad common test case name

Currently you get a crash message like:

```
%%% otel_metrics_SUITE ==> unnamed_instrument: FAILED
%%% otel_metrics_SUITE ==> {undef,
    [{otel_metrics_SUITE,unnamed_instrument,
         [[{watchdog,<0.1676.0>},
```

If you try to run a test that doesn't exist. It'd be better if we could handle this `undef` and print a message.