Tell how to resume a channel switch that failed partway

omarchy-channel-set runs under set -e, so a failure after it has begun
mutating the system (dev link, pacman channel, packages) died silently
with the switch half-applied. Trap ERR once the mutation phase starts
and say how to pick the switch back up.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

bin/omarchy-channel-set

@@ -72,6 +72,10 @@ case "$channel" in
     ;;
 esac
 
+# A failure past this point leaves the channel switch half-applied, so say how
+# to pick it back up rather than dying silently under set -e.
+trap 'echo -e "\nThe channel switch did not complete. Review the error above, then rerun: omarchy-channel-set '"$channel"'" >&2' ERR
+
 if [[ -z $dev_checkout && $OMARCHY_PATH != "/usr/share/omarchy" ]]; then
   leaving_dev=1
 fi