Add ShouldNeverHappen exception

cheuph/exceptions.py

@@ -1,4 +1,10 @@
-__all__ = ["MessageSupplyException"]
+__all__ = ["MessageSupplyException", "ShouldNeverHappen"]
 
 class MessageSupplyException(Exception):
     pass
+
+class ShouldNeverHappen(Exception):
+    def __init__(self, number: int) -> None:
+        message = (f"SNV{number:05} - please contact @Garmy with the code on"
+                " the left if you see this")
+        super().__init__(message)