Revert "hooks/fancy: disable handle_output for the time being."

This reverts commit 6c8d33b00ee106a0ff3d0574c525b0afb5e5e35f.
hooks/fancy: handle_output: ignore TERM/HUP/INT signals.

This allows to Ctrl+C the running script properly if it intercepts
TERM/HUP/INT on its own.
hooks/fancy: report_success, report_fail: deduplicate code.
hooks/fancy: handle_output: play nice to `set -e`: use if statement instead of $?.
hooks/fancy: reduce impact of passing stderr via handler by doing this only between BUSY and DONE/FAILED lines.