Include requestMethod in RPC remote error message.
Prevent calling callback twice.

Return after the call to quit() to prevent the
callback from being called twice.
Allow 'internetexplorer' as a browser name.

For launching IE on buggy Selenium Hubs that don't accept the
standard "Internet Explorer" (with a space). This will be reverted
once we no longer need to launch on these buggy Hubs.
Nice errors for WebDriver launching.