Disable tracebacks on IIIF import logging

Part of #108 (closed). The logging messages already include the image URLs; the only actionable item is to remove the tracebacks, which add clutter to the logging messages. It turns out the tracebacks are automatically added by the logging module when sys.exc_info is available, and the simplest way to disable them is to do so in each log call by adding exc_info=False.