add SAML signature validation errors to log output
add SAML signature validation errors to log output

fixes: CNVS-13402

Test Plan:
  Create a failing SAML signature validation and make sure it outputs
  the validation error message to the log.

Example:
  Failed to verify SAML signature: Invalid fingerprint (expected
  #{expected_fingerprint}, got #{fingerprint})

Change-Id: I2960c256ffbaf782e51a71bbecb3adbbddac9f64
Reviewed-on: https://gerrit.instructure.com/36420
Tested-by: Jenkins <d95b56ce41a2e1ac4cecdd398defd7414407cc08@instructure.com>
Reviewed-by: Brian Palmer <3cb4e1df5ec4da2c7c4af7c52cec8cf340a55a10@instructure.com>
QA-Review: August Thornton <9adc7a1161ddf32ff608de792a7e50179545f026@instructure.com>
Product-Review: Nick Cloward <9cc571ed222c7ca8c35f018ff48a612f8cadeb73@instructure.com>

juoni/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,efrj/canvas-lms,shidao-fm/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sdeleon28/canvas-lms,ShreniiNepal/shrenii_lmp,greyhwndz/canvas-lms,narigone/canvas,leftplusrightllc/canvas-lms,HotChalk/canvas-lms,eCNAP/CANVAS,tgroshon/canvas-lms,wye0220/shindig-canvas,matematikk-mooc/canvas-lms,sfu/canvas-lms,snehakachroo1/LSI_replica,SwinburneOnline/canvas-lms,redconfetti/canvas-lms,utkarshx/canvas-lms,utkarshx/canvas-lms,ShreniiNepal/shrenii_lmp,kyroskoh/canvas-lms,matematikk-mooc/canvas-lms,sdeleon28/canvas-lms,whalejasmine/canvas-lms,dgynn/canvas-lms,snehakachroo1/LSI_replica,instructure/canvas-lms,HotChalk/canvas-lms,whalejasmine/canvas-lms,antoniuslin/canvas-lms,juoni/canvas-lms,efrj/canvas-lms,wye0220/shindig-canvas,instructure/canvas-lms,dgynn/canvas-lms,djbender/canvas-lms,leftplusrightllc/canvas-lms,jay3126/canvas-lms,instructure/canvas-lms,Rvor/canvas-lms,sdeleon28/canvas-lms,narigone/canvas,jay3126/canvas-lms,antoniuslin/canvas-lms,utkarshx/canvas-lms,redconfetti/canvas-lms,HotChalk/canvas-lms,shidao-fm/canvas-lms,roxolan/canvas-lms,ShreniiNepal/shrenii_lmp,sfu/canvas-lms,SwinburneOnline/canvas-lms,dgynn/canvas-lms,va7map/canvas-lms,whalejasmine/canvas-lms,samuels410/greatlakes-lms,fronteerio/canvas-lms,skmezanul/canvas-lms,eCNAP/CANVAS,redconfetti/canvas-lms,leftplusrightllc/canvas-lms,Unow/canvas-lms,tgroshon/canvas-lms,wrdsb/canvas-lms,kyroskoh/canvas-lms,Unow/canvas-lms,tgroshon/canvas-lms,greyhwndz/canvas-lms,shidao-fm/canvas-lms,roxolan/canvas-lms,fronteerio/canvas-lms,wrdsb/canvas-lms,venturehive/canvas-lms,venturehive/canvas-lms,roxolan/canvas-lms,snehakachroo1/LSI_replica,Rvor/canvas-lms,HotChalk/canvas-lms,tgroshon/canvas-lms,fronteerio/canvas-lms,va7map/canvas-lms,skmezanul/canvas-lms,Rvor/canvas-lms,samuels410/greatlakes-lms,instructure/canvas-lms,antoniuslin/canvas-lms,AndranikMarkosyan/lms,efrj/canvas-lms,skmezanul/canvas-lms,sfu/canvas-lms,fronteerio/canvas-lms,venturehive/canvas-lms,matematikk-mooc/canvas-lms,juoni/canvas-lms,SwinburneOnline/canvas-lms,AndranikMarkosyan/lms,skmezanul/canvas-lms,samuels410/greatlakes-lms,redconfetti/canvas-lms,roxolan/canvas-lms,djbender/canvas-lms,Unow/canvas-lms,shidao-fm/canvas-lms,djbender/canvas-lms,wrdsb/canvas-lms,grahamb/canvas-lms,snehakachroo1/LSI_replica,Jyaasa/canvas-lms,whalejasmine/canvas-lms,matematikk-mooc/canvas-lms,Jyaasa/canvas-lms,Jyaasa/canvas-lms,kyroskoh/canvas-lms,venturehive/canvas-lms,djbender/canvas-lms,grahamb/canvas-lms,greyhwndz/canvas-lms,wrdsb/canvas-lms,utkarshx/canvas-lms,eCNAP/CANVAS,instructure/canvas-lms,eCNAP/CANVAS,jay3126/canvas-lms,va7map/canvas-lms,instructure/canvas-lms,grahamb/canvas-lms,AndranikMarkosyan/lms,narigone/canvas,juoni/canvas-lms,Jyaasa/canvas-lms,gbleydon/canvas-lms,efrj/canvas-lms,greyhwndz/canvas-lms,antoniuslin/canvas-lms,dgynn/canvas-lms,leftplusrightllc/canvas-lms,SwinburneOnline/canvas-lms,grahamb/canvas-lms,va7map/canvas-lms,kyroskoh/canvas-lms,ShreniiNepal/shrenii_lmp,jay3126/canvas-lms,sdeleon28/canvas-lms,samuels410/greatlakes-lms,gbleydon/canvas-lms,gbleydon/canvas-lms,grahamb/canvas-lms,sfu/canvas-lms,Rvor/canvas-lms,AndranikMarkosyan/lms,wye0220/shindig-canvas,narigone/canvas