applied patch from Malcolm Tredinnick fixing python exception hierarchy

* python/libxml.py: applied patch from Malcolm Tredinnick fixing
  python exception hierarchy #139824
Daniel
diff --git a/python/libxml.py b/python/libxml.py
index 1036bd9..588e862 100644
--- a/python/libxml.py
+++ b/python/libxml.py
@@ -1,28 +1,31 @@
 import libxml2mod
 import types
 
+# The root of all libxml2 errors.
+class libxmlError(Exception): pass
+
 #
 # Errors raised by the wrappers when some tree handling failed.
 #
-class treeError:
+class treeError(libxmlError):
     def __init__(self, msg):
         self.msg = msg
     def __str__(self):
         return self.msg
 
-class parserError:
+class parserError(libxmlError):
     def __init__(self, msg):
         self.msg = msg
     def __str__(self):
         return self.msg
 
-class uriError:
+class uriError(libxmlError):
     def __init__(self, msg):
         self.msg = msg
     def __str__(self):
         return self.msg
 
-class xpathError:
+class xpathError(libxmlError):
     def __init__(self, msg):
         self.msg = msg
     def __str__(self):