[Bug] When pysqa is not installed

[jan-janssen]

The following example from the documentation gives a cryptic error message:

def add_funct(a, b):
    return a + b


from executorlib import FluxClusterExecutor

with FluxClusterExecutor(cache_directory="./file") as exe:
    future = 0
    for i in range(4, 8):
        future = exe.submit(add_funct, i, future)
    print(future.result())

The error message is:

Exception in thread Thread-5 (execute_tasks_h5):
Traceback (most recent call last):
  File "/srv/conda/envs/notebook/lib/python3.12/threading.py", line 1075, in _bootstrap_inner
    self.run()
  File "/srv/conda/envs/notebook/lib/python3.12/site-packages/ipykernel/ipkernel.py", line 766, in run_closure
    _threading_Thread_run(self)
  File "/srv/conda/envs/notebook/lib/python3.12/threading.py", line 1012, in run
    self._target(*self._args, **self._kwargs)
  File "/srv/conda/envs/notebook/lib/python3.12/site-packages/executorlib/task_scheduler/file/shared.py", line 131, in execute_tasks_h5
    process_dict[task_key] = execute_function(
                             ^^^^^^^^^^^^^^^^^
  File "/srv/conda/envs/notebook/lib/python3.12/site-packages/executorlib/task_scheduler/file/subprocess_spawner.py", line 48, in execute_in_subprocess
    raise ValueError("backend parameter is not supported for subprocess spawner.")
ValueError: backend parameter is not supported for subprocess spawner.

[jan-janssen]

In addition, I should also check the error messages when other dependencies like h5py are not installed.