Improve serializable dict

[tensionhead]

Changes Summary

• pass through error messages of json.dumps
• try the _serialize_value helper before testing serializability
• added test

Reviewer Checklist

• [ ] Are testing routines present?
• [ ] Do objects in the global package namespace perform proper parsing of their input?
• [ ] Are all docstrings complete and accurate?
• [ ] Is the CHANGELOG.md up to date?

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.14% :warning:

Comparison is base (9b1d86b) 69.10% compared to head (200b541) 68.97%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## dev #569 +/- ## ========================================== - Coverage 69.10% 68.97% -0.14% ========================================== Files 87 87 Lines 10213 10217 +4 Branches 2151 2153 +2 ========================================== - Hits 7058 7047 -11 - Misses 2559 2574 +15 Partials 596 596 ``` | [Files Changed](https://app.codecov.io/gh/esi-neuroscience/syncopy/pull/569?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=esi-neuroscience) | Coverage Δ | | |---|---|---| | [syncopy/shared/tools.py](https://app.codecov.io/gh/esi-neuroscience/syncopy/pull/569?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=esi-neuroscience#diff-c3luY29weS9zaGFyZWQvdG9vbHMucHk=) | `86.71% <100.00%> (+0.42%)` | :arrow_up: | ... and [3 files with indirect coverage changes](https://app.codecov.io/gh/esi-neuroscience/syncopy/pull/569/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=esi-neuroscience)

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