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.