Update assert_failure.bash

Author: jakub-bochenski

src/assert_failure.bash

@@ -71,6 +71,9 @@ assert_failure() {
       'actual'   "$status"
       batslib_print_kv_single_or_multi "$width" \
       'output' "$output"
+      if [[ -n "$stderr" ]]; then
+      	batslib_print_kv_single_or_multi "$width" 'stderr' "$stderr"
+      fi
     } \
     | batslib_decorate 'command failed as expected, but status differs' \
     | fail