Generate better error messages when encountering unexpected i128/u128 values during deserialization

[jonasbb]

Tests are omitted since they do not work with serde_json. They are treated as large floats.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 28 lines in your changes missing coverage. Please review.

Project coverage is 65.84%. Comparing base (4c273b2) to head (e8d2e02).
Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
serde_with/src/utils.rs	0.00%	18 Missing ⚠️
serde_with/src/de/impls.rs	0.00%	10 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #810      +/-   ##
==========================================
- Coverage   66.40%   65.84%   -0.56%     
==========================================
  Files          40       40              
  Lines        2625     2647      +22     
==========================================
  Hits         1743     1743              
- Misses        882      904      +22     

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