lib/dialog: Log the actual error message, not the object
lib/dialog: Log the actual error message, not the object

Only strings will make it all the way through our testing machinery,
and we need to see them at the other end for detecting known issues.

Closes #9963
martinpitt/cockpit,martinpitt/cockpit,andreasn/cockpit,cockpit-project/cockpit,moraleslazaro/cockpit,deryni/cockpit,moraleslazaro/cockpit,martinpitt/cockpit,deryni/cockpit,deryni/cockpit,deryni/cockpit,andreasn/cockpit,moraleslazaro/cockpit,mvollmer/cockpit,cockpituous/cockpit,garrett/cockpit,cockpituous/cockpit,cockpit-project/cockpit,garrett/cockpit,mvollmer/cockpit,cockpit-project/cockpit,moraleslazaro/cockpit,moraleslazaro/cockpit,cockpituous/cockpit,andreasn/cockpit,martinpitt/cockpit,andreasn/cockpit,andreasn/cockpit,mvollmer/cockpit,cockpituous/cockpit,deryni/cockpit,deryni/cockpit,martinpitt/cockpit,cockpituous/cockpit,mvollmer/cockpit,garrett/cockpit,mvollmer/cockpit,andreasn/cockpit,moraleslazaro/cockpit,andreasn/cockpit,cockpit-project/cockpit,garrett/cockpit,deryni/cockpit,garrett/cockpit,moraleslazaro/cockpit,cockpit-project/cockpit,cockpituous/cockpit