[SUGGESTION] Scanpy find markers step can fail when only one cluster found from clustering algorithm

[dweemx]

Is your feature request related to a problem? Please describe.
Pipeline can fail at the Scanpy find markers step when clustering resolution is too low which leads to having a single cluster. Here, Leiden resolution 0.2 lead to 1 cluster.

Describe the solution you'd like
Error raised is not human readable:

Command error:
  /opt/venv/lib/python3.7/site-packages/louvain/Optimiser.py:349: SyntaxWarning: assertion is always true, perhaps remove parentheses?
    assert(issubclass(partition_type, LinearResolutionParameterVertexPartition),
  Traceback (most recent call last):
    File "/staging/leuven/stg_00002/lcb/dwmax/documents/aertslab/GitHub/vib-singlecell-nf/vsn-pipelines/src/scanpy/bin/cluster/sc_marker_genes.py", line 114, in <module>
      n_genes=ngenes
    File "/opt/venv/lib/python3.7/site-packages/scanpy/tools/_rank_genes_groups.py", line 367, in rank_genes_groups
      mean_rest, var_rest = _get_mean_var(X[mask_rest])  # for fold-change
    File "/opt/venv/lib/python3.7/site-packages/scanpy/preprocessing/_utils.py", line 10, in _get_mean_var
      mean = X.mean(axis=0)
    File "/opt/venv/lib/python3.7/site-packages/scipy/sparse/base.py", line 1091, in mean
      return (inter_self * (1.0 / self.shape[0])).sum(
  ZeroDivisionError: float division by zero

Describe alternatives you've considered
We should provide a better error to the user.

Additional context
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