Should not panic when older browsers do not provide error objects for `onerror()`

[Dennis Lin]

Ref: https://blog.sentry.io/2016/01/04/client-javascript-reporting-window-onerror.html

in server.js:

onerror = function(m,s,l,c,e) {
  if (e.__handled) return;

For some clients (e.g., Safari 9.x) the 5th param e - the error object does not present. We should at least not throw in the meanwhile.