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