Gracefully handle missing and invalid idtoken

[michael-doubez]

Plugin should not assume provider is compliant:

• missing idtoken should be reported with relevant localized error message.
• username field should never be null (default to "sub") and if not found, localized message to be displayed

Testing done

Submitter checklist

Preview Give feedback

1. Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
2. Ensure that the pull request title represents the desired changelog entry
3. Please describe what you did
4. Link to relevant issues in GitHub or Jira
5. Link to relevant pull requests, esp. upstream and downstream changes
6. Ensure you have provided tests - that demonstrates feature works or fixes the issue

[michael-doubez]

Fixes #213

[codecov]

Codecov Report

Attention: Patch coverage is 82.75862% with 5 lines in your changes are missing coverage. Please review.

Project coverage is 73.71%. Comparing base (c3d9244) to head (ed221ad).
Report is 4 commits behind head on master.

Files	Patch %	Lines
...va/org/jenkinsci/plugins/oic/OicSecurityRealm.java	88.88%	3 Missing ⚠️
...va/org/jenkinsci/plugins/oic/OicTokenResponse.java	0.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #292      +/-   ##
============================================
+ Coverage     73.16%   73.71%   +0.54%     
- Complexity      189      194       +5     
============================================
  Files             9        9              
  Lines           775      814      +39     
  Branches        113      115       +2     
============================================
+ Hits            567      600      +33     
- Misses          151      157       +6     
  Partials         57       57              

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