Include Kea response in the toast

The issue was found by @slawek during 1.19 sanity checks: #1519 (comment 495429)

The toast message doesn't contain the Kea response, so it is hard to recognize what happened.

Logs:

server-1                  | time="2024-10-01 11:29:13" level="error" msg="Problem with committing Kea config" file="           config.go:780  " error="config-set command to kea@agent-kea failed: error status (1) returned by Kea dhcp4 daemon with text: ''cache-threshold' parameter is not a real'"