test if in nbdev_repo & improve nbdev_export err message

[hamelsmu]

@jph00 IIUC you suggested making this function public

def _nbdev_config_file(cfg_name=_nbdev_cfg_name, path=None):
    cfg_path = path = Path.cwd() if path is None else Path(path)
    while cfg_path != cfg_path.parent and not (cfg_path/cfg_name).exists(): cfg_path = cfg_path.parent
    if not (cfg_path/cfg_name).exists(): cfg_path = path
    return cfg_path/cfg_name

But you still have to check if that path exists. The way to check if nbdev exists might not be as apparent to users so maybe a simple function that helps you check (which is named appropriately) is better, given its usefulness?

Also, I improved the error message with nbdev_export

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[hamelsmu]

maybe you just want to use nbdev_config_file().exists() directly instead?

[jph00]

Looks like CI problems @hamelsmu ! :D

[jph00]

@hamelsmu did you want to mark this draft, or is it ready?

[hamelsmu]

My jupyter notebook got angry with the indentation here. So I fixed it FYI

[hamelsmu]

@jph00 it is ready now 😄