Reading Location files gives unhelpful error

I have managed to either corrupt my location files or my index.csv in such a way that this error appears. Unfortunately it is quite unhelpful since one does not know which files are missing or where the problem lies.

Ideally, DP would check if location files exist and include the missing file or values in the error message.

```
None
{'BRD-K68867920-051-11-5': 0, 'BRD-K74363950-004-01-0': 1, 'BRD-K75958547-238-01-0': 2, 'DMSO': 3}
Reading single-cell locations
Traceback (most recent call last):
File "/usr/lib/python3.6/runpy.py", line 193, in _run_module_as_main
"__main__", mod_spec)
File "/usr/lib/python3.6/runpy.py", line 85, in _run_code
exec(code, run_globals)
File "deepprofiler/__main__.py", line 197, in
cli(obj={})
File "/usr/local/lib/python3.6/dist-packages/click/core.py", line 1137, in __call__
return self.main(*args, **kwargs)
File "/usr/local/lib/python3.6/dist-packages/click/core.py", line 1062, in main
rv = self.invoke(ctx)
File "/usr/local/lib/python3.6/dist-packages/click/core.py", line 1668, in invoke
return _process_result(sub_ctx.command.invoke(sub_ctx))
File "/usr/local/lib/python3.6/dist-packages/click/core.py", line 1404, in invoke
return ctx.invoke(self.callback, **ctx.params)
File "/usr/local/lib/python3.6/dist-packages/click/core.py", line 763, in invoke
return __callback(*args, **kwargs)
File "/usr/local/lib/python3.6/dist-packages/click/decorators.py", line 26, in new_func
return f(get_current_context(), *args, **kwargs)
File "deepprofiler/__main__.py", line 162, in train
dset = deepprofiler.dataset.image_dataset.read_dataset(context.obj["config"], mode='train')
File "/DeepProfiler/deepprofiler/dataset/image_dataset.py", line 243, in read_dataset
dset.prepare_training_locations()
File "/DeepProfiler/deepprofiler/dataset/image_dataset.py", line 74, in prepare_training_locations
locations = pd.concat(locations)
File "/usr/local/lib/python3.6/dist-packages/pandas/core/reshape/concat.py", line 284, in concat
sort=sort,
File "/usr/local/lib/python3.6/dist-packages/pandas/core/reshape/concat.py", line 331, in __init__
raise ValueError("No objects to concatenate")
ValueError: No objects to concatenate
```