Add SAXReaderNotAvailable, and use it to distinguish between an
ImportError, and a missing driver.
diff --git a/Lib/xml/sax/expatreader.py b/Lib/xml/sax/expatreader.py
index 2fc2b7c..eb46bcd 100644
--- a/Lib/xml/sax/expatreader.py
+++ b/Lib/xml/sax/expatreader.py
@@ -6,7 +6,10 @@
 version = "0.20"
 
 from xml.sax._exceptions import *
-from xml.parsers import expat
+try:
+    from xml.parsers import expat
+except ImportError:
+    raise SAXReaderNotAvailable("expat not supported",None) 
 from xml.sax import xmlreader, saxutils, handler
 
 AttributesImpl = xmlreader.AttributesImpl