Make middleware.py Python 2.6 compatible

Fixes bug 1206256

In Python 2.6 xml.etree.ElementTree.fromstring raises
xml.parsers.expat.ExpatError instead of
xml.etree.ElementTree.ParseError when faced with invalid xml.
This change catches an import error and uses ExpatError instead of
ParseError.

Change-Id: I8ab41328c8eeacc28f4e01d730c73ff251df4a25

ceilometer/api/middleware.py

@@ -24,6 +24,10 @@ Based on pecan.middleware.errordocument
 import json
 import webob
 from xml import etree as et
+try:
+    from xml.etree.ElementTree import ParseError
+except ImportError:
+    from xml.parsers.expat import ExpatError as ParseError
 
 from ceilometer.openstack.common import log
 
@@ -76,7 +80,7 @@ class ParsableErrorMiddleware(object):
                             et.ElementTree.fromstring('<error_message>'
                                                       + '\n'.join(app_iter)
                                                       + '</error_message>'))]
-                except et.ElementTree.ParseError as err:
+                except ParseError as err:
                     LOG.error('Error parsing HTTP response: %s' % err)
                     body = ['<error_message>%s' % state['status_code']
                             + '</error_message>']