Update PostgresError's DatabaseError conformance for new PostgresNIO behavior

[gwynne]

This makes DatabaseError.isSyntaxError and .isConstraintFailure work again.

[codecov-commenter]

Codecov Report

Merging #208 (ec340a6) into main (c7cd020) will decrease coverage by 0.75%. The diff coverage is 23.80%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #208 +/- ## ========================================== - Coverage 77.29% 76.55% -0.75% ========================================== Files 7 7 Lines 414 354 -60 ========================================== - Hits 320 271 -49 + Misses 94 83 -11 ``` | [Impacted Files](https://app.codecov.io/gh/vapor/fluent-postgres-driver/pull/208?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=vapor) | Coverage Δ | | |---|---|---| | [.../FluentPostgresDriver/PostgresError+Database.swift](https://app.codecov.io/gh/vapor/fluent-postgres-driver/pull/208?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=vapor#diff-U291cmNlcy9GbHVlbnRQb3N0Z3Jlc0RyaXZlci9Qb3N0Z3Jlc0Vycm9yK0RhdGFiYXNlLnN3aWZ0) | `76.92% <23.80%> (-9.19%)` | :arrow_down: | ... and [6 files with indirect coverage changes](https://app.codecov.io/gh/vapor/fluent-postgres-driver/pull/208/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=vapor)

[VaporBot]

These changes are now available in 2.6.2