Test should be aware of error on image read.

* tests/test_parse.py (test_read_image_missing): Subject.
Add a "close" button to the derand preview window.

* util/derandui.py (_showPrev): Subject.
Keep track of the current directory in derandui

* util/derandui.py (self._currDir): New variable to hold the last directory that
the user chose in the QFileDialog interaction.
Be more specific about what exception we expect

We expect RIException to be raised, not just any exception