Improve feedback when misconfiguring a strategy

[PeterJCLaw]

Currently if you misconfigure a strategy, perhaps by omitting a name or by forgetting to call super().__init__() passing everything through in its initialiser, devdata doesn't give much useful feedback.
The error you do get is:

  File ".../devdata/engine.py", line 72, in export_data
    model_strategies = sort_model_strategies(settings.strategies)
  File ".../devdata/utils.py", line 72, in sort_model_strategies
    for dep in strategy.depends_on:
AttributeError: 'tuple' object has no attribute 'depends_on'

Which is unfortunately considerably later in devdata's processing.

I think the fix is likely to change the handling in settings.py, so that the strategy construction logic is more explicitly able to tell the difference between the available valid options and those which are erroneous:

django-devdata/src/devdata/settings.py

Lines 39 to 44 in da8933c

	try:
	klass_path, kwargs = strategy
	klass = import_string(klass_path)
	ret[app_model_label].append(klass(**kwargs))
	except (ValueError, TypeError, IndexError):
	ret[app_model_label].append(strategy)