Clarify user-facing string to remove internal terminology

The user doesn't know what a global stack is. This is a message that more clearly specifies what is really happening as far as the user can see.

The global stack may be None if this code is executed while there is no active printer. Normally the only time there is no active printer is:
- during first launch, while in the welcome screen
- during start-up until the profiles have been loaded
- if there was a profile corruption of some kind with the active printer.

So this function should not get called during those times. It probably isn't called except for the last case (in which case it's acceptable I guess).
This change is made after a request from a translator what the hell this means.

Contributes to issue CURA-7783.

cura/Settings/CuraContainerRegistry.py

@@ -440,7 +440,7 @@ class CuraContainerRegistry(ContainerRegistry):
 
         global_stack = cura.CuraApplication.CuraApplication.getInstance().getGlobalContainerStack()
         if not global_stack:
-            return False, catalog.i18nc("@info:status", "Global stack is missing.")
+            return False, catalog.i18nc("@info:status", "There is no active printer yet.")
 
         definition_id = ContainerTree.getInstance().machines[global_stack.definition.getId()].quality_definition
         profile.setDefinition(definition_id)