Fixes #952: Improved exc message when CloseEnumeration context is None

Signed-off-by: Andreas Maier <maiera@de.ibm.com>

pywbem/cim_operations.py

@@ -6032,7 +6032,9 @@ def CloseEnumeration(self, context, **extra):
             # entire context is tested for None because the open/pull methods
             # set the complete context to None when eos received.
             if context is None:
-                raise ValueError('None is not a valid EnumerationContext')
+                raise ValueError("Invalid context: None "
+                                 "(Enumeration may be exhausted)")
+
             self._imethodcall(
                 method_name,
                 namespace=context[1],