feat(saml): improve error handling

[nijel]

Catch SAML error and raise standard AuthFailed error from it.

[codecov]

Codecov Report

❌ Patch coverage is 60.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.98%. Comparing base (2b84c69) to head (0a49b7c).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
social_core/backends/saml.py	60.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1545      +/-   ##
==========================================
- Coverage   78.99%   78.98%   -0.02%     
==========================================
  Files         354      354              
  Lines       11509    11513       +4     
  Branches      526      526              
==========================================
+ Hits         9092     9094       +2     
- Misses       2236     2238       +2     
  Partials      181      181              

Flag	Coverage Δ
unittests	78.98% <60.00%> (-0.02%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.