Slighly refactored ControllerAdvices to include handling of Spring's
HTTP Exceptions. This fixes 500 status codes being sent instead of the
correct one.

Fixes GH-67.

Backport of d052fd73.

Fixes regression introduced by commit
c88cce27.

fix exporting all answers on session export

See merge request !93

Migration version now falls back to -1 if no document exists.