v0.2.0

fix: correctly implement error interfaces on wrapped errors

There were two problems:

* `ExpectedError` and `UnexpectedError` should unwrap themselves
* `ErrorSet` should support equivalency check when there's a single
error inside.

This fixes the scenario of matching an error from a retry loop (in case if
just a single error was encountered).

Signed-off-by: Andrey Smirnov <smirnov.andrey@gmail.com>