function ProxyErrorHandler() {
  // Handle proxy error events.
  chrome.proxy.onProxyError.addListener(this.handleError_.bind(this));

  // Handle message events from popup.
  chrome.extension.onRequest.addListener(this.handleOnRequest_.bind(this));
}
The proxy error handling object. Binds to the 'onProxyError' event, and
changes the extensions badge to reflect the error state (yellow for
non-fatal errors, red for fatal).

@constructor