frontend: change "no such script" to "unknown command"

as far as the user is concerned, 'foo' in `lb foo` is a command; they do
not know or need to know that the commands also correspond to actual
scripts.

Gbp-Dch: Short

frontend/lb

@@ -55,7 +55,7 @@ elif [ $(which "${COMMAND}") ]; then
 	# User has live-build commands in path
 	SCRIPT="${COMMAND}"
 else
-	Echo_error "No such script: ${COMMAND}"
+	Echo_error "Unknown command: ${COMMAND}"
 	exit 1
 fi