It is inconsistent because none of the other methods throw this exception (create, update), even though they take the same parameters
Not really sure why the method changes the url string to URL and back to String ? No other method does this. Is that by accident or for a reason?
In any case, even if there is a reason, I think the exception treatment should be internalised. The URL is provided by the user at the creation of the restmanager and will then be verified anyways
https://github.com/geosolutions-it/geoserver-manager/blob/master/src/main/java/it/geosolutions/geoserver/rest/manager/GeoServerRESTStoreManager.java#L103