Use modern Python exception syntax

"except Exception as e" instead of "except Exception, e"

This is part of a transition to supporting Python 3.  Python >= 2.6
support "as" syntax.

Note: this removes Python 2.5 support.

Change-Id: I309599f3981bba2b46111c43102bee38ff132803

manifest_xml.py

@@ -336,7 +336,7 @@ class XmlManifest(object):
             # tricky.  actual parsing implementation may vary.
             except (KeyboardInterrupt, RuntimeError, SystemExit):
                 raise
-            except Exception, e:
+            except Exception as e:
                 raise ManifestParseError(
                     "failed parsing included manifest %s: %s", (name, e))
         else: