python3: use 'as' syntax for exception assignment

From http://docs.python.org/3.1/whatsnew/3.0.html

The except exc as var syntax is now standard and except exc,
var is no longer supported.

Change-Id: Ia3a11d72c474806cc2303a7fc9cb1a390266c0fb
Signed-off-by: Chuck Short <chuck.short@canonical.com>
Merge "python3: use 'as' syntax for exception assignment"