Move exceptions to their own file

Since that keeps the GlobalStack cleaner

Contributes to CURA-3497

cura/Settings/Exceptions.py

@@ -0,0 +1,17 @@
+# Copyright (c) 2017 Ultimaker B.V.
+# Cura is released under the terms of the AGPLv3 or higher.
+
+
+##  Raised when trying to perform an operation like add on a stack that does not allow that.
+class InvalidOperationError(Exception):
+    pass
+
+
+##  Raised when trying to replace a container with a container that does not have the expected type.
+class InvalidContainerError(Exception):
+    pass
+
+
+##  Raised when trying to add an extruder to a Global stack that already has the maximum number of extruders.
+class TooManyExtrudersError(Exception):
+    pass

cura/Settings/GlobalStack.py

@@ -11,8 +11,7 @@ from UM.Settings.InstanceContainer import InstanceContainer
 from UM.Settings.DefinitionContainer import DefinitionContainer
 from UM.Settings.ContainerRegistry import ContainerRegistry
 
-class CannotSetNextStackError(Exception):
+from . import Exceptions
-    pass
 
 class GlobalStack(ContainerStack):
     def __init__(self, container_id: str, *args, **kwargs):
@@ -78,7 +77,7 @@ class GlobalStack(ContainerStack):
     ##  Overridden from ContainerStack
     @override(ContainerStack)
     def setNextStack(self, next_stack: ContainerStack) -> None:
-        raise CannotSetNextStackError("Global stack cannot have a next stack!")
+        raise Exceptions.InvalidOperationError("Global stack cannot have a next stack!")
 
     ##  Overridden from ContainerStack
     @override(ContainerStack)