handle ROOTError (illegal number of points) when saving canvas (issue #77)

I tried to fix issue #77 with this patch. Here I am intercepting ROOTError with message " illegal number of points" when saving a canvas. I don't see a better solution since there is no hierarchy of ROOTError exceptions, apart from intercepting all the ROOTErros.

The bearmer presentation works now.

In addition now canvases with no points are also displayed.

![download](https://f.cloud.github.com/assets/143389/273412/a327faf8-9022-11e2-855f-7dae66898c46.png)