srgb curve grades dialog

fixes CNVS-12784

test plan:
 * in srgb
 * on an assignment with no submissions
 - open the curve grades dialog
 - click 'Curve Grades'
 > a little error box should show up
 > the page should not blow up

 * in gb2
 * do the same thing as above
 > it should show the error box and not blow up

Change-Id: I8a60d37bcc424746497313f766a381be3a6b175a
Reviewed-on: https://gerrit.instructure.com/34803
Reviewed-by: Liz Abinante <labinante@instructure.com>
Product-Review: Cameron Sutter <csutter@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Amber Taniuchi <amber@instructure.com>