Add Pool custom-error APIs and blocking client variants

[ryanfowler]

Summary

• Added the missing Pool::conn_and_then and Pool::conn_mut_and_then methods, delegating through self.get() to match the Client API.
• Added blocking custom-error variants on Client and Pool: conn_and_then_blocking and conn_mut_and_then_blocking.
• Covered the new API surface with async and blocking regression tests that exercise custom error propagation.

Testing

• cargo test
• cargo clippy -- -D warnings
• cargo fmt --all -- --check