shell: add timeout logic and rework error handling

what's important:

* if task code ran, it exits with 0. this code is regardless of (error, result)
  * when it exited cleanly, we will get the values from the database

* if task timed out, the box code kills it and it has a flag tracking timedOut. we can
  ignore exit code in this case.

* if task code was stopped, box code will send SIGTERM which ideally it will handle and end with 70.

* if task code crashed and it caught the exception, it will return 50

* if task code crashed and node nuked us, it will exit with 1

* if task code was killed with some unhandleabe signal, taskworker.sh will return the signal (9=SIGKILL)

src/scripts/kill-child.sh

@@ -23,6 +23,8 @@ function killtree() {
         killtree "${cpid}" || true
     done
     echo "kill-child: killing $pid"
+    kill -SIGTERM "${pid}" 2>/dev/null || true
+    sleep 1
     kill -SIGKILL "${pid}" 2>/dev/null || true
 }