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 <jenkins@instructure.com>
Reviewed-by: Brian Palmer <brianp@instructure.com>
QA-Review: August Thornton <august@instructure.com>
Product-Review: Nick Cloward <ncloward@instructure.com>