fix error handling

lib/bouncer/backends/v0/accept.go

lib/bouncer/bouncers/v2/bouncer.go

lib/bouncer/frontends/v0/accept.go

lib/gat/pooler.go

lib/gat/pools/session/pool.go

lib/gat/pools/transaction/conn.go

lib/gat/pools/transaction/pool.go

lib/zap/reader.go

lib/zap/readwriter.go

lib/zap/writer.go