handle crocodoc/canvadocs timeouts when creating sessions

closes CNVS-16086

Test plan:
  * do something to make api calls to crocodoc time out
  * try to preview a crocodoc document
  * you should see an error that suggests you try again later
  * repeat for canvadocs

Change-Id: I26619cbbe95940cf51027fb8ea50a7c727722c83
Reviewed-on: https://gerrit.instructure.com/42389
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Amber Taniuchi <amber@instructure.com>
Product-Review: Cameron Matheson <cameron@instructure.com>
Reviewed-by: Josh Simpson <jsimpson@instructure.com>