Count a variable as skipped if its dependencies are skipped

[takluyver]

At present, a dependent variable is recorded as skipped if its dependencies return None, but as having an error if its dependencies raise Skip:

With this change, the Skip is propagated instead, making it easier to see which variables actually hit an error:

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.32%. Comparing base (734ab7b) to head (b7fd714).
⚠️ Report is 5 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #561      +/-   ##
==========================================
- Coverage   79.34%   79.32%   -0.02%     
==========================================
  Files          42       42              
  Lines        7659     7658       -1     
==========================================
- Hits         6077     6075       -2     
- Misses       1582     1583       +1     

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[tmichela]

Suggested change

	if dep_errors and not all(isinstance(e, Skip) for _, e in dep_errors):
	errors[name] = DependencyError(dep_errors)
	if dep_errors:
	errors[name] = DependencyError(dep_errors)
	if all(isinstance(e, Skip) for _, e in dep_errors):
	errors[name] = Skip(str(errors[name]))

I suggest to keep building the dependency tree of skipped variables, to help understand why it was skipped.