I have added tests and fixed one or two bugs in 1405df4a1535.  I have another patch that checks that error messages are useful, with one exception: if you try to import a.b and b raises an ImportError, then the calling code will see an Attribute error.  Right now I don’t see how we can avoid that; checking the existence of files like your patch proposes is not possible, as Python modules do not come from files.