Send slack message on failure
Send slack message on failure

closes EVAL-1055
flag=none

test plan:
  - jenkins build sends slack message on failure
  - jenkins build does not fail with unbound BUILD_REGISTRY_FQDN variable

Change-Id: I5fd97f446ced03b6097f6eb6c77cd8eefe6ed1d3
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/243466
Tested-by: Service Cloud Jenkins <9144042a601061f88f1e1d7a1753ea3e2972119d@instructure.com>
Reviewed-by: Adrian Packel <9c6a39807293376f29cba17542b4cb1b663ecc4a@instructure.com>
Reviewed-by: Gary Mei <4dff5ebcff9ed76793b70b9470ac64791a16ed11@instructure.com>
QA-Review: Syed Hussain <1379c89cd1d253d3c5a299cc4d8f6002ada9d6b5@instructure.com>
Product-Review: Syed Hussain <1379c89cd1d253d3c5a299cc4d8f6002ada9d6b5@instructure.com>

instructure/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,sfu/canvas-lms