Includes the expression of the condition in the assertion message if
the assertion failed, making assertions much easier to debug. eg.
error: assertion (withPython -> (python2Packages != null)) failed at pkgs/tools/security/nmap/default.nix:11:1
(cherry picked from commit 307bcb9a8e)
Signed-off-by: Domen Kožar <domen@dev.si>