Better error for non-existing modules in submodules

[Carreau]

Config files allow to define unlisted submodules for which to gather docs, example from scikit-image:

[global]
submodules = [ "morphology", "color", "data", "draw", "exposure", "io", "graph", "segmentation", "transform", "util", "viewer",]

If a submodule does not exists, there is a generic error that does not point to it being from the configuration (example "viewer" from above)

Traceback (most recent call last):
  File "/Users/bussonniermatthias/miniforge3/envs/papyri/bin/papyri", line 8, in <module>
    sys.exit(app())
             ^^^^^
  File "/Users/bussonniermatthias/miniforge3/envs/papyri/lib/python3.11/site-packages/typer/main.py", line 328, in __call__
    raise e
  File "/Users/bussonniermatthias/miniforge3/envs/papyri/lib/python3.11/site-packages/typer/main.py", line 311, in __call__
    return get_command(self)(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/bussonniermatthias/miniforge3/envs/papyri/lib/python3.11/site-packages/click/core.py", line 1157, in __call__
    return self.main(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/bussonniermatthias/miniforge3/envs/papyri/lib/python3.11/site-packages/typer/core.py", line 778, in main
    return _main(
           ^^^^^^
  File "/Users/bussonniermatthias/miniforge3/envs/papyri/lib/python3.11/site-packages/typer/core.py", line 216, in _main
    rv = self.invoke(ctx)
         ^^^^^^^^^^^^^^^^
  File "/Users/bussonniermatthias/miniforge3/envs/papyri/lib/python3.11/site-packages/click/core.py", line 1688, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/bussonniermatthias/miniforge3/envs/papyri/lib/python3.11/site-packages/click/core.py", line 1434, in invoke
    return ctx.invoke(self.callback, **ctx.params)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/bussonniermatthias/miniforge3/envs/papyri/lib/python3.11/site-packages/click/core.py", line 783, in invoke
    return __callback(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/bussonniermatthias/miniforge3/envs/papyri/lib/python3.11/site-packages/typer/main.py", line 683, in wrapper
    return callback(**use_params)  # type: ignore
           ^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/bussonniermatthias/dev/papyri/papyri/__init__.py", line 460, in gen
    gen_main(
  File "/Users/bussonniermatthias/dev/papyri/papyri/gen.py", line 550, in gen_main
    g.collect_api_docs(target_module_name, limit_to=limit_to)
  File "/Users/bussonniermatthias/dev/papyri/papyri/gen.py", line 1902, in collect_api_docs
    collector: DFSCollector = self._get_collector()
                              ^^^^^^^^^^^^^^^^^^^^^
  File "/Users/bussonniermatthias/dev/papyri/papyri/gen.py", line 1770, in _get_collector
    nx = __import__(name)
         ^^^^^^^^^^^^^^^^
ModuleNotFoundError: No module named 'skimage.viewer'

Maybe we should special case that with a better error message.