title: PM2 not restarting clusters on errors
body: <p>When I use PM2 to run multiple processes (i.e. cluster mode) and one of those processes encounters an uncaught error, PM2 does not restart that process.<br>
  Why?<br>
  How do I make it restart workers in cluster mode?</p>
  
  <p><strong>Example code</strong></p>
  
  <pre><code>// index.js
  let counter = 0;
  setInterval(function(){
    if(counter &gt;= 5) {
      throw new Error('Worker crash. Why no restart?');
    }
    counter++;
    console.log('Worker alive: ' + Date.now() );
  },500);
  </code></pre>
  
  <p><strong>Run on the cmd line</strong></p>
  
  <pre><code>pm2 start index.js -i 4
  pm2 log
  </code></pre>
  
  <p>Eventually all the workers crash and never restart.<br>
  Whats the point of restarting if it can only be done on a single process.</p>
  
  <p><strong>pm2 logs (merged into single file)</strong></p>
  
  <pre><code>    Worker alive: 1522937847186
  Worker alive: 1522937847231
  Worker alive: 1522937847276
  Worker alive: 1522937847324
  Worker alive: 1522937847691
  Worker alive: 1522937847736
  Worker alive: 1522937847781
  Worker alive: 1522937847830
  Worker alive: 1522937848193
  Worker alive: 1522937848238
  Worker alive: 1522937848283
  Worker alive: 1522937848332
  Worker alive: 1522937848693
  Worker alive: 1522937848738
  Worker alive: 1522937848783
  Worker alive: 1522937848832
  Worker alive: 1522937849194
  Worker alive: 1522937849238
  Worker alive: 1522937849284
  Worker alive: 1522937849333
  Error: Worker crash. Why no restart?
      at Timeout._onTimeout (/home/usrname/docs/Projects_NodeJS/project/app/index.js:49:11)
      at ontimeout (timers.js:466:11)
      at tryOnTimeout (timers.js:304:5)
      at Timer.listOnTimeout (timers.js:267:5)
  Error: Worker crash. Why no restart?
      at Timeout._onTimeout (/home/usrname/docs/Projects_NodeJS/project/app/index.js:49:11)
      at ontimeout (timers.js:466:11)
      at tryOnTimeout (timers.js:304:5)
      at Timer.listOnTimeout (timers.js:267:5)
  Error: Worker crash. Why no restart?
      at Timeout._onTimeout (/home/usrname/docs/Projects_NodeJS/project/app/index.js:49:11)
      at ontimeout (timers.js:466:11)
      at tryOnTimeout (timers.js:304:5)
      at Timer.listOnTimeout (timers.js:267:5)
  Error: Worker crash. Why no restart?
      at Timeout._onTimeout (/home/usrname/docs/Projects_NodeJS/project/app/index.js:49:11)
      at ontimeout (timers.js:466:11)
      at tryOnTimeout (timers.js:304:5)
      at Timer.listOnTimeout (timers.js:267:5)
  </code></pre>