Example of exception

[1.17747] >> gnuplot --version
Traceback (most recent call last):
  File "./gitstats", line 1373, in <module>
      g.run(sys.argv[1:]
  File "./gitstats", line 1365, in run
     report.create(data, outputpath)
  File "./gitstats", line 841, in create
     f.write('<tr><td>%s</td><td>%d</td><td>%d</td><td>%d</td></tr>' % (yymm, data.commits_by_month[yymm], data.lines_added_by_month[yymm],
   data.lines_removed_by_month[yymm]))
KeyError: '2010-08'

Signed-off-by: Heikki Hokkanen <hoxu@users.sf.net>