Changed Warning message and return for non existent labs

pyiricdata/Client.py

@@ -364,8 +364,8 @@ class Client:
     """ Check if lab is available """
     def _check_lab_exists(self, lab):
         if self.labs is None:
-            sys.stderr.write('WARNING: Not able to check lab existence')
-            lab = None
+            sys.stderr.write("WARNING: Not able to check lab existence when using personal authentication token")
+            return None
 
         lab = str(lab) if lab is not None else lab
         if lab != '' and lab is not None: