add native logging to check_upward_messages (#1906)

* add native logging to check_upward_messages

This doesn't affect the WASM builds, but ensures that native versions
(such as are used for testing) emit diagnostics anytime the
check_upward_messages function exits, which should reduce the pain
of debugging when something goes wrong.

* Apply suggestions from code review

Co-authored-by: Sergei Shulepov <sergei@parity.io>

* verb tense

Co-authored-by: Sergei Shulepov <sergei@parity.io>