improve response for downloading an xform with errors

Previously, when a download url for an xform within a working version of
an app is hit, it would 500. Now it 404s, with a body containing the
same errors that are displayed on the form-view page.

For downloading the xforms with a _build_, however, the behavior remains
unchanged.
raise a more intentional error when a bind is missing

fixes a test
Merge branch 'master' into xform-error-404
Merge pull request #2073 from dimagi/xform-error-404

improve response for downloading an xform with errors