Improve Serialize test to check classes in Protocol name space separately .
Fix Serializable::unserialize() for Announcement.
Improve error handling by passing ErrorImportance enumeration within Error template. Also there is new signal called "ProcessingFailure" introduced to D-Bus interfaces (with corresponding wrapper DBus::emit_processing_failure_signal()).
Mark DBus::emit_processing_failure_signal() as noexcept to use safely in catch statements.
Serializable::unserialize() methods now check if provided buffer has enough data to unserialize object.
Improve error handling in Neighbourhood and mark Neighbourhood::handle_announcement_register_noexcept()/handle_announcement_unregister_noexcept() methods as noexcept.