Change unsupported Selenium driver error message

[martinmladenov]

Andy only supports the HtmlUnitDriver and doesn't support other drivers such as FirefoxDriver, ChromeDriver, etc. The error message currently says "Please use HtmlUnitDriver as Andy does not have a browser installed". This may be a bit confusing to students, as in exercises we always use a facade for the driver logic (CSE1110.createDriver()) so it might be better to change the error message to refer to this method instead. For example: "Please use CSE1110.createDriver() to instantiate a Selenium driver".