UI should prevent creating multiple namespaces with same URI

Creating multiple namespaces with the same URI causes GeoServer problems (having to do with reverse lookup, and the one-to-many relationship therein). This seems to make sense to me, and so rather than suggest that GeoServer operate otherwise, I propose that we prevent this from happening, perhaps by the UI returning an error saying "this URI already exists, please try another one" much in the same way that the same shapefile can't be registered as two different featureTypes.