RUM-4359: Add status code in user-facing message in case of UnknownError during batch upload

[0xnm]

What does this PR do?

This change aims to help in investigation of #2013: it will be handy to have HTTP status code logged in the user-facing message in case of UnknownError during batch upload.

Review checklist (to be filled by reviewers)

• Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• Make sure you discussed the feature or bugfix with the maintaining team in an Issue
• Make sure each commit and the PR mention the Issue number (cf the CONTRIBUTING doc)

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.81%. Comparing base (2adb85c) to head (6d10b15).

Additional details and impacted files

@@                Coverage Diff                @@
##           release/2.9.0    #2018      +/-   ##
=================================================
- Coverage          83.92%   83.81%   -0.11%     
=================================================
  Files                488      488              
  Lines              17781    17744      -37     
  Branches            2667     2662       -5     
=================================================
- Hits               14922    14871      -51     
- Misses              2154     2161       +7     
- Partials             705      712       +7     

Files	Coverage Δ
.../android/core/internal/data/upload/UploadStatus.kt	98.21% <100.00%> (ø)

... and 24 files with indirect coverage changes