A Python error can have links to other errors by means of both .Unwrap()
and .__cause__ . These ways are both explicit and so should be treated
by e.g. errors.Is as present in error's error chain.

It is a bit unclear, at least initially, how to linearise and order
error chain traversal in divergence points - for exception objects where
both .Unwrap() and .__cause__ are !None. However more closer look
suggests linearisation rule to traverse into .__cause__ after going
through .Unwrap() part - please see details in documentation added into
_error.pyx

-> Teach errors.Is to do this traversal, and this way now e.g. exception
raised as

	raise X from Y

will be treated by errors.Is as being both X and Y, even if any of X or Y
also has its own error chain via .Unwrap().

Top-level documentation is TODO.