Ensure error messages thrown by data decoding provide useful information

[gwynne]

These changes are now available in 2.15.1

Under some conditions, a failed attempt to decode data from an SQLRow (whether via SQLKit or through Fluent) would throw errors whose descriptions were missing information critical to understanding the problem. This fixes that.

Also adds some useful notes about using PostgresKit with PostgresClient to the README.

[codecov]

Codecov Report

❌ Patch coverage is 71.42857% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 42.84%. Comparing base (2fc0afb) to head (5fe62d3).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
Sources/PostgresKit/PostgresDataTranslation.swift	71.42%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #277      +/-   ##
==========================================
+ Coverage   42.30%   42.84%   +0.53%     
==========================================
  Files          13       13              
  Lines         851      852       +1     
==========================================
+ Hits          360      365       +5     
+ Misses        491      487       -4     

Files with missing lines	Coverage Δ
Sources/PostgresKit/PostgresDataTranslation.swift	68.77% <71.42%> (+1.60%)	⬆️

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[MahdiBM]

LGTM