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 :white_check_mark:

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

Additional details and impacted files

```diff @@ 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](https://app.codecov.io/gh/DataDog/dd-sdk-android/pull/2018?dropdown=coverage&src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=DataDog) | Coverage Δ | | |---|---|---| | [.../android/core/internal/data/upload/UploadStatus.kt](https://app.codecov.io/gh/DataDog/dd-sdk-android/pull/2018?src=pr&el=tree&filepath=dd-sdk-android-core%2Fsrc%2Fmain%2Fkotlin%2Fcom%2Fdatadog%2Fandroid%2Fcore%2Finternal%2Fdata%2Fupload%2FUploadStatus.kt&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=DataDog#diff-ZGQtc2RrLWFuZHJvaWQtY29yZS9zcmMvbWFpbi9rb3RsaW4vY29tL2RhdGFkb2cvYW5kcm9pZC9jb3JlL2ludGVybmFsL2RhdGEvdXBsb2FkL1VwbG9hZFN0YXR1cy5rdA==) | `98.21% <100.00%> (ø)` | | ... and [24 files with indirect coverage changes](https://app.codecov.io/gh/DataDog/dd-sdk-android/pull/2018/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=DataDog)