Handle TypeError from table column summation code

This commit catches TypeError from horizon.tables.Column
summation calculation. This TypeError is caught inside Django
until Django 1.6, but Django 1.7 code is refined to catch
more specific case and it leads to horizon unit test failure.

Closes-Bug: #1355939
Change-Id: I9d5b4565f1238a9880ccf117f2ea623fed466a44
Merge "Handle TypeError from table column summation code"