Make matplotlib.test() print meaninful messages when baseline images are not installed

Reported originally in the mailing list
http://www.mail-archive.com/matplotlib-users@lists.sourceforge.net/msg27140.html

After installing matplotlib from standard Ubuntu packages, calling matplotlib.test() results in a lot of errors like:
```
IOError: Baseline image
'/home/user/result_images/test_triangulation/tripcolor1-expected.svg' does not exist.
```
This seems to be because baseline images are not included in the package (is it also required to run the tests from a particular directory?). I guess it happens in other distros and with windows installers too.