Add PSQLError debugDescription

[gwynne]

Fixes #354, vapor/postgres-nio#380. Continues work from #356 and #360.

Requires minor version bump (adds new public API in the form of protocol conformances).

[codecov-commenter]

Codecov Report

Merging #372 (b172872) into main (fcb2e66) will increase coverage by 1.36%. The diff coverage is 67.16%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #372 +/- ## ========================================== + Coverage 42.98% 44.35% +1.36% ========================================== Files 118 118 Lines 8243 8629 +386 ========================================== + Hits 3543 3827 +284 - Misses 4700 4802 +102 ``` | [Impacted Files](https://app.codecov.io/gh/vapor/postgres-nio/pull/372?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=vapor) | Coverage Δ | | |---|---|---| | [Sources/PostgresNIO/Data/PostgresDataType.swift](https://app.codecov.io/gh/vapor/postgres-nio/pull/372?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=vapor#diff-U291cmNlcy9Qb3N0Z3Jlc05JTy9EYXRhL1Bvc3RncmVzRGF0YVR5cGUuc3dpZnQ=) | `56.19% <54.91%> (-4.19%)` | :arrow_down: | | [Sources/PostgresNIO/New/PostgresQuery.swift](https://app.codecov.io/gh/vapor/postgres-nio/pull/372?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=vapor#diff-U291cmNlcy9Qb3N0Z3Jlc05JTy9OZXcvUG9zdGdyZXNRdWVyeS5zd2lmdA==) | `78.51% <84.05%> (+4.54%)` | :arrow_up: | | [Sources/PostgresNIO/New/PSQLError.swift](https://app.codecov.io/gh/vapor/postgres-nio/pull/372?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=vapor#diff-U291cmNlcy9Qb3N0Z3Jlc05JTy9OZXcvUFNRTEVycm9yLnN3aWZ0) | `75.27% <88.37%> (+20.48%)` | :arrow_up: |