Allow to propagate errors in order to handle them acordingly.
Use playerNotifications by value to reduce the amount of allocations.
Add session managment playerNotifications.
Support marshalling notifications to plain text.

This allows to not use the internal game state directly from client
code.

TODO: * select the variadic components
      * test the costs type

Also make more state methods private.

Panicf includes a stacktrace and causes a debugger to halt.