fix "development only" error flash details link
fix "development only" error flash details link

fixes CNVS-16940

test plan: Run in a beta / test / dev instance, and make an ajax request fail.
The easiest way I've found to do this is as follows:
1. Conclude a course (course settings | "Conclude this course" button)
2. Go to the concluded course's People page
3. Click the "Add users" button (it looks disabled, but it isn't; this is a bug, but not the bug this commit attempts to fix)
4. Try to enroll a user. On the last page of the wizard, you should see a "That request didn't work out" flash message
   with a "details..." link below it that opens another dialog that gives the ajax request error code.
   (you should see the link, not the HTML code for the link)

Change-Id: Ie999f753021db55d916f6e5c6cdba3eaffdf7e72
Reviewed-on: https://gerrit.instructure.com/44529
Tested-by: Jenkins <d95b56ce41a2e1ac4cecdd398defd7414407cc08@instructure.com>
Reviewed-by: Clay Diffrient <9dff2e5c98626d20d2848250c411e8009465bb87@instructure.com>
Product-Review: Dan Minkevitch <2591e5f46f28d303f9dc027d475a5c60d8dea17a@instructure.com>
QA-Review: Jahnavi Yetukuri <ed2b2672ccc2e59e56c0615df7f01b579202e014@instructure.com>

snehakachroo1/LSI_replica,venturehive/canvas-lms,juoni/canvas-lms,AndranikMarkosyan/lms,HotChalk/canvas-lms,SwinburneOnline/canvas-lms,Jyaasa/canvas-lms,shidao-fm/canvas-lms,eCNAP/CANVAS,kyroskoh/canvas-lms,kyroskoh/canvas-lms,antoniuslin/canvas-lms,efrj/canvas-lms,redconfetti/canvas-lms,HotChalk/canvas-lms,roxolan/canvas-lms,wrdsb/canvas-lms,juoni/canvas-lms,sdeleon28/canvas-lms,instructure/canvas-lms,narigone/canvas,instructure/canvas-lms,sfu/canvas-lms,tgroshon/canvas-lms,snehakachroo1/LSI_replica,narigone/canvas,AndranikMarkosyan/lms,samuels410/greatlakes-lms,samuels410/greatlakes-lms,instructure/canvas-lms,sfu/canvas-lms,AndranikMarkosyan/lms,Rvor/canvas-lms,venturehive/canvas-lms,djbender/canvas-lms,eCNAP/CANVAS,snehakachroo1/LSI_replica,roxolan/canvas-lms,venturehive/canvas-lms,va7map/canvas-lms,dgynn/canvas-lms,sfu/canvas-lms,juoni/canvas-lms,va7map/canvas-lms,djbender/canvas-lms,leftplusrightllc/canvas-lms,fronteerio/canvas-lms,antoniuslin/canvas-lms,grahamb/canvas-lms,SwinburneOnline/canvas-lms,leftplusrightllc/canvas-lms,sfu/canvas-lms,dgynn/canvas-lms,shidao-fm/canvas-lms,grahamb/canvas-lms,jay3126/canvas-lms,instructure/canvas-lms,samuels410/greatlakes-lms,utkarshx/canvas-lms,venturehive/canvas-lms,Rvor/canvas-lms,fronteerio/canvas-lms,matematikk-mooc/canvas-lms,sfu/canvas-lms,utkarshx/canvas-lms,utkarshx/canvas-lms,grahamb/canvas-lms,va7map/canvas-lms,Rvor/canvas-lms,Jyaasa/canvas-lms,ShreniiNepal/shrenii_lmp,HotChalk/canvas-lms,whalejasmine/canvas-lms,djbender/canvas-lms,skmezanul/canvas-lms,kyroskoh/canvas-lms,fronteerio/canvas-lms,narigone/canvas,instructure/canvas-lms,sfu/canvas-lms,ShreniiNepal/shrenii_lmp,kyroskoh/canvas-lms,dgynn/canvas-lms,whalejasmine/canvas-lms,eCNAP/CANVAS,grahamb/canvas-lms,leftplusrightllc/canvas-lms,tgroshon/canvas-lms,efrj/canvas-lms,grahamb/canvas-lms,juoni/canvas-lms,roxolan/canvas-lms,jay3126/canvas-lms,sdeleon28/canvas-lms,AndranikMarkosyan/lms,eCNAP/CANVAS,skmezanul/canvas-lms,redconfetti/canvas-lms,matematikk-mooc/canvas-lms,redconfetti/canvas-lms,antoniuslin/canvas-lms,instructure/canvas-lms,narigone/canvas,greyhwndz/canvas-lms,jay3126/canvas-lms,Rvor/canvas-lms,whalejasmine/canvas-lms,leftplusrightllc/canvas-lms,redconfetti/canvas-lms,roxolan/canvas-lms,whalejasmine/canvas-lms,instructure/canvas-lms,Jyaasa/canvas-lms,Jyaasa/canvas-lms,antoniuslin/canvas-lms,shidao-fm/canvas-lms,shidao-fm/canvas-lms,tgroshon/canvas-lms,greyhwndz/canvas-lms,greyhwndz/canvas-lms,efrj/canvas-lms,jay3126/canvas-lms,wrdsb/canvas-lms,sdeleon28/canvas-lms,va7map/canvas-lms,djbender/canvas-lms,skmezanul/canvas-lms,efrj/canvas-lms,wrdsb/canvas-lms,SwinburneOnline/canvas-lms,samuels410/greatlakes-lms,ShreniiNepal/shrenii_lmp,SwinburneOnline/canvas-lms,ShreniiNepal/shrenii_lmp,fronteerio/canvas-lms,greyhwndz/canvas-lms,tgroshon/canvas-lms,HotChalk/canvas-lms,sdeleon28/canvas-lms,matematikk-mooc/canvas-lms,matematikk-mooc/canvas-lms,wrdsb/canvas-lms,utkarshx/canvas-lms,dgynn/canvas-lms,snehakachroo1/LSI_replica,skmezanul/canvas-lms,sfu/canvas-lms