Handle error when ecommerce-service-uri is not configured

[jhorsman]

When the ecommerce-service-uri key is not present in the web.config the webapp will log this error:

2016-11-29 21:43:39,450 [7] ERROR - Error executing child request for handler 'System.Web.Mvc.HttpHandlerUtil+ServerExecuteHttpHandlerAsyncWrapper'.System.Web.HttpException (0x80004005): Error executing child request for handler 'System.Web.Mvc.HttpHandlerUtil+ServerExecuteHttpHandlerAsyncWrapper'. ---> System.UriFormatException: Invalid URI: The format of the URI could not be determined. but that is probably config

This could be handled a bit nicer with a more specific error message.