Issue #42

Make list() and dict() work as zero parameter constructors
Issue 40:

Make the error message in the browser more meaningful when the
file (i.e. div) cannot be openned.

There is no unit test because it is browser-only code.

During unit tests, open() calls turn into Sk.read() which handles missing
files properly already.
Merge pull request #44 from csev/master

Issues 40 and 42 - Improve open() message, make list() and dict() work.