resolve unresolved error nits of #7617 (#7631)

* handle executor should_panic test better

* Revert "reduce should panic, due to extended error messages"

This reverts commit c0805940184a62cd9302603ad911c3591e70a60c.

* remove excessive constraints

* remove duplicate documentation messages for error variants

* reduce T: constraints to the abs minimum

* whoops

* fewer bounds again

Co-authored-by: Bernhard Schuster <bernhard@parity.io>