Fix format handling so it works with py3.4

Nose-progressive fails while building with python 3.4 in fedora:
http://kojipkgs.fedoraproject.org//work/tasks/5198/6855198/build.log
because of line_number is None when formatting

Python 3.3:
>>> line = None
>>> line_max = 10
>>> '+{line_number:<{line_number_max_width}}'.format(line_number=line, line_number_max_width=line_max)
'+None 

Python 3.4:
>>> line = None
>>> line_max = 10
>>> '+{line_number:<{line_number_max_width}}'.format(line_number=line, line_number_max_width=line_max)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: non-empty format string passed to object.__format__


See http://bugs.python.org/issue7994