Feature: Add exclusion violation error kind

[barskern]

A rather trival addition, though there are other error codes which could probably be considered aswell. It seems as though the selection of error kinds has been kept small for a reason, i.e. only add error kind for the most "normal" errors a user would want to programmatically handle, do something with. For my usecase, the exclusion violation fits this premise, and I would say it's destinct enough from the unique violation to deserve it's own kind. Anyhow, thank you for a great library, and feel free to do with this PR what you want! 😄

Does your PR solve an issue?

No.

Is this a breaking change?

No, the enum is marked as non_exhaustive, so adding a variant is backwards compatible.

[abonander]

Would you please add a test here: https://github.com/launchbadge/sqlx/blob/main/tests/postgres/error.rs

Also, is this applicable to the other drivers?

[barskern]

Would you please add a test here: https://github.com/launchbadge/sqlx/blob/main/tests/postgres/error.rs

Of course, tried to grep for tests with that enum but didn't really find it, so thanks for pointing me to it!

Also, is this applicable to the other drivers?

I am not too familiar with the other drivers, though from some googling it seems as though both SQLite and MSSQL doesn't have it (they rather have check constraints it seems).