We recently changed a variable from diff_info_or_response to

diff_info (02608ae) in our diff fragment code, but some of the error

handling blocks still referenced the old variable name. This change

fixes that. The commit that introduced the new variable name hasn't

shipped so we don't need to worry about backporting this.

Viewed an error message displayed in a diff fragment in the diff

  viewer. Before this change, that was broken.
Ran unit tests.