I couldn't find a way to set my own "Invalid Email address" page as I can with the other landing pages. I would like to suggest the possibilty to do this directly through the CampaignMonitor interface, if it isn't there already. :)
It isn't possible to edit the default error. Your best bet is to add on your own custom validation (something like this) and then you can define the error display and text to be whatever you want.
I would like to suggest adding this as a feature then though. ;)