Create shipment if it doesn't exist; exit with error if update flag not set.

- Create shipment if it doesn't exist, when the shipment and update flag are both not set.
- Print an error message and exit with error code if the "How did you get here?" case is reached.

src/trackbert/__main__.py

@@ -145,7 +145,7 @@ def main():
             print(f"Shipment {args.tracking_number} does not exist. Remove -u to create.")
             exit(1)
 
-        else:
+        if not shipment and not args.update:
             tracker.db.create_shipment(
                 args.tracking_number, args.carrier, args.description
             )
@@ -155,6 +155,9 @@ def main():
 
             exit(0)
 
+        print("How did you get here?")
+        exit(1)
+
     if args.tracking_number is not None:
         if args.disable:
             if not tracker.db.get_shipment(args.tracking_number):