photoidx.py create raises KeyError if exiftags are not present in an image

[RKrahl]

When creating a new index, photoidx.py create reads some exif tags like DateTimeOriginal and Orientaion to add the information to the index. If some tag is not present in any of the images, a KeyError is raised:

$ photoidx.py create
Traceback (most recent call last):
  ...
KeyError: 'Exif.Photo.DateTimeOriginal: Unknown tag'

This should be handled more gracefully.