refs #3999 the network client should really not output any error message and also not handle any error
refs #3999 performance improvement by fetching only the website ids, not all site information
refs #3999 display any network errors, display the all websites dashboard even if user has access to only one website --> need to test how Piwik itself handles this case, performance improvement by doing less requests and using the MultiSites.getAll API directly and not MetadataAPI because this one needs no siteId, simplified the controller in general
refs #3999 trigger error handler if response does not have valid format, simplified the adapter
refs #3999 display network error messages and simplified/refactored some parts