Catch errors in client.js; show in html reporter

- The window.onerror handler is now installed in client.js instead of
  client.html.
- The message displayed for uncaught errors is appended to the DOM
  rather than written over it.
- The html reporter has been updated to display uncaught errors.
- The window.onerror handler in realClient is now registered after
  reporters load, leaving the one from client.js operational until that
  point.

refs #310