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/test/tasks-test.js

@@ -121,6 +121,17 @@ describe('task', function () {
         const [error, result] = await safe(tasks.startTask(taskId, {}));
         if (!error) throw new Error('expecting task to stop');
         expect(error.message).to.contain('stopped');
+        expect(error.code).to.be(tasks.ESTOPPED);
+        expect(result).to.not.be.ok();
+    });
+
+    it('task timesout', async function () {
+        const taskId = await tasks.add(tasks._TASK_SLEEP, [ 10000 ]);
+
+        const [error, result] = await safe(tasks.startTask(taskId, { timeout: 2000 }));
+        if (!error) throw new Error('expecting task to timeout');
+        expect(error.code).to.be(tasks.ETIMEOUT);
+        expect(error.message).to.contain('timed out');
         expect(result).to.not.be.ok();
     });
 });