Add support to provide more information for UnexpectedPageException

[leonard84]

Currently we only get the class name of the unexpected page as information,e.g.:
geb.error.UnexpectedPageException: An unexpected page com.acme.errors.ErrorPage was encountered when trying to find page match (given potentials: [class com.acme.HomePage, class com.acme.LoginPage])

In some instance it would be helpful to have the option to include additional information here.
I propose adding a new interface UnexpectedPage that has the method String getUnexpectedPageMessage() (feel free to use a better name) which is called after the page matches the at-checker. The String would then be included in the UnexpectedPageException message. Alternatively geb.Page could be extended with this method and provide a default implementation that returns null/"".

I know that there is the PageEventListener, but I think that this use-case still has merits, especially since you can't modify the exception message.