Fix audit file timestamp.

* timestream/manipulate/pipecomponents.py
(ResultingFeatureWriter.__chkExcept__): We write the correct timestamp instead
of NaN.
Propagate Corrupt Image Error.

* timestream/manipulate/__init__.py (PCExCorruptImage): New error. Should be
triggered when image.pixels throws an exception.
* timestream/manipulate/pipecomponents.py: Enclose all first pixel references in
a "try..catch" so we can throw the correct PCExCorruptImage exception.
* timestream/parse/__init__.py (read_image, RIException): Propagate the
corrupt image error from read_image. We should find a better place for both the
error class and the read_image method.