Extend PostgresCastingError

[fabianfett]

I forgot to add some important metadata to the PostgresCastingError. For this reason here it is... Also added some API comments that will come in handy once we make this type public.

[codecov-commenter]

Codecov Report

Merging #221 (e6b90cc) into main (112a5e5) will increase coverage by 4.85%.
The diff coverage is 42.85%.

@@            Coverage Diff             @@
##             main     #221      +/-   ##
==========================================
+ Coverage   42.96%   47.81%   +4.85%     
==========================================
  Files         119      119              
  Lines        8219     8217       -2     
==========================================
+ Hits         3531     3929     +398     
+ Misses       4688     4288     -400     

Flag	Coverage Δ
unittests	47.81% <42.85%> (+4.85%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
Sources/PostgresNIO/New/PSQLError.swift	70.96% <42.85%> (+13.22%)	⬆️
...urces/PostgresNIO/New/Data/Array+PSQLCodable.swift	94.68% <0.00%> (+1.06%)	⬆️
...rces/PostgresNIO/New/Extensions/Logging+PSQL.swift	19.60% <0.00%> (+1.42%)	⬆️
Sources/PostgresNIO/New/PSQLConnection.swift	31.68% <0.00%> (+1.98%)	⬆️
...s/PostgresNIO/Message/PostgresMessageDecoder.swift	95.45% <0.00%> (+2.27%)	⬆️
...es/PostgresNIO/New/PSQLBackendMessageDecoder.swift	93.28% <0.00%> (+2.98%)	⬆️
...urces/PostgresNIO/New/Messages/ErrorResponse.swift	32.30% <0.00%> (+3.07%)	⬆️
...urces/PostgresNIO/New/Data/Float+PSQLCodable.swift	76.66% <0.00%> (+3.33%)	⬆️
Sources/PostgresNIO/New/PSQLBackendMessage.swift	96.63% <0.00%> (+3.36%)	⬆️
...ources/PostgresNIO/Data/PostgresData+Numeric.swift	70.44% <0.00%> (+3.64%)	⬆️
... and 19 more

[gwynne]

🎉 ❤️