Made MetadataDateTimeError be public.
Made MetadataDateTimeError be public.

This is needed for automatic tests that can exercise functionality that
can throw this error. 
ritchiewilson/shotwell,Sanva/shotwell-gsoc,polymeris/shotwell,h4ck3rm1k3/shotwell,polymeris/shotwell,h4ck3rm1k3/shotwell,h4ck3rm1k3/shotwell,polymeris/shotwell,jimbonator/shotwell,jof/shotwell,jof/shotwell,damianmoore/shotwell,Sanva/shotwell-gsoc,damianmoore/shotwell,jimbonator/shotwell,abrauchli/shotwell,damianmoore/shotwell,jof/shotwell,jimbonator/shotwell,Sanva/shotwell-gsoc,abrauchli/shotwell,ritchiewilson/shotwell,ritchiewilson/shotwell,abrauchli/shotwell