[Miyao] Sometimes the job fails with always the same error (don't know why)

The summary of the error is this:

```
Exception: Gamera cannot find staves with threshold blackness 0.3
```

I don't understand what that means!

The long error is this:
```Traceback (most recent call last):
File "/usr/local/lib/python3.7/site-packages/celery/app/trace.py", line 412, in trace_task
R = retval = fun(*args, **kwargs)
File "/usr/local/lib/python3.7/site-packages/celery/app/trace.py", line 704, in __protected_call__
return self.run(*args, **kwargs)
File "/code/Rodan/rodan/jobs/base.py", line 791, in run
retval = self.run_my_task(inputs, settings, arg_outputs)
File "/code/Rodan/rodan/jobs/heuristic_pitch_finding/base.py", line 77, in run_my_task
staves = sf.get_staves(image)
File "/code/Rodan/rodan/jobs/heuristic_pitch_finding/StaffFinding.py", line 37, in get_staves
self._find_staves(s)
File "/code/Rodan/rodan/jobs/heuristic_pitch_finding/StaffFinding.py", line 267, in _find_staves
raise Exception("Gamera cannot find staves with threshold blackness 0.3")
Exception: Gamera cannot find staves with threshold blackness 0.3
```

I can't reliably reproduce this error, and I can't figure out what triggers it. I'm just creating this issue because if anyone knows what that threshold blackness is I would _really_ love to know.