Incorrect error when selector is incorrect

[jasonb5]

[2020-10-14 18:57:08,292: ERROR/ForkPoolWorker-7] Task compute_tasks.cdat.task_subset[dd7af05e-75b1-45ee-a81a-c0179a14e313] raised unexpected: IndexError('tuple index out of range')
Traceback (most recent call last):
  File "/opt/conda/lib/python3.7/site-packages/xarray/core/dataset.py", line 1164, in _construct_dataarray
    variable = self._variables[name]
KeyError: 'lat'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/opt/conda/lib/python3.7/site-packages/compute_tasks/cdat.py", line 748, in process_subset
    before = input.coords[dim.name].shape
  File "/opt/conda/lib/python3.7/site-packages/xarray/core/coordinates.py", line 214, in __getitem__
    return cast("DataArray", self._data[key])
  File "/opt/conda/lib/python3.7/site-packages/xarray/core/dataset.py", line 1254, in __getitem__
    return self._construct_dataarray(key)
  File "/opt/conda/lib/python3.7/site-packages/xarray/core/dataset.py", line 1167, in _construct_dataarray
    self._variables, name, self._level_coords, self.dims
  File "/opt/conda/lib/python3.7/site-packages/xarray/core/dataset.py", line 167, in _get_virtual_variable
    ref_var = variables[ref_name]
KeyError: 'lat'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/opt/conda/lib/python3.7/site-packages/compute_tasks/cdat.py", line 716, in build_workflow
    output = build_process(context, next, interm)
  File "/opt/conda/lib/python3.7/site-packages/compute_tasks/cdat.py", line 691, in build_process
    output = process._process_func(context, next, *inputs, **params)
  File "/opt/conda/lib/python3.7/site-packages/compute_tasks/cdat.py", line 761, in process_subset
    raise WPSError('Unable to select to select data with {!r}', selector)
compute_tasks.WPSError: Unable to select to select data with {'lat': slice(-20, 20, 1)}

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/opt/conda/lib/python3.7/site-packages/celery/app/trace.py", line 412, in trace_task
    R = retval = fun(*args, **kwargs)
  File "/opt/conda/lib/python3.7/site-packages/celery/app/trace.py", line 704, in __protected_call__
    return self.run(*args, **kwargs)
  File "/opt/conda/lib/python3.7/site-packages/celery/app/base.py", line 490, in run
    return task._orig_run(*args, **kwargs)
  File "/opt/conda/lib/python3.7/site-packages/compute_tasks/cdat.py", line 1263, in task_subset
    return workflow(context)
  File "/opt/conda/lib/python3.7/site-packages/compute_tasks/base.py", line 260, in _inner_wrapper
    return func(*args, **kwargs)
  File "/opt/conda/lib/python3.7/site-packages/compute_tasks/cdat.py", line 1069, in workflow
    interm = build_workflow(context)
  File "/opt/conda/lib/python3.7/site-packages/compute_tasks/cdat.py", line 722, in build_workflow
    raise WPSError(f'Failed to build process {next.name}: {repr(e)}')
  File "/opt/conda/lib/python3.7/site-packages/compute_tasks/__init__.py", line 11, in __init__
    self.msg = fmt.format(*args, **kwargs)
IndexError: tuple index out of range

Formatting error hides true error in WPSException resposne.