Queries fail on pool when postgres is out of connection slots

**Describe the bug**
I'm not sure this is a bug or intended behavior.

I used up all the connection slots on my Postgres server (by accident), and then the queries I tried to run on my pgxpool returned:

`FATAL: remaining connection slots are reserved for non-replication superuser connections (SQLSTATE 53300)`

I expected this error to be hidden if there were still open connections in the pool so that my application can continue running without noticing any problems.

(However, maybe that error should be returned once the context given to Pool.Acquire() gets cancelled, as that would likely be the actual failure reason?)

**To Reproduce**
Steps to reproduce the behavior:

I suspect this will show the failure, but didn't test it.

```go
package main

import (
"context"
"fmt"
"log"
"os"

"github.com/jackc/pgx/v5"
"github.com/jackc/pgx/v5/pgxpool"
)

func main() {
pool, err := pgxpool.Connect(context.Background(), os.Getenv("DATABASE_URL"))
if err != nil {
log.Fatal(err)
}
var connections []*pgx.Conn
for {
// Exhaust all connection slots.
conn, err := pgx.Connect(context.Background(), os.Getenv("DATABASE_URL"))
if err != nil {
break
}
connections = append(connections, conn)
}
_, err := pool.Exec("SELECT 1")
fmt.Print(err)
}

```

**Expected behavior**
I'd like the pool to hide errors creating new connections while we have at least MinConns open connections.

**Actual behavior**
One of the queries on my pool failed with:

`FATAL: remaining connection slots are reserved for non-replication superuser connections (SQLSTATE 53300)`

**Version**
- Go: `go version go1.18.something linux/amd64`
- PostgreSQL: `$ psql --no-psqlrc --tuples-only -c 'select version()'` -> [e.g. PostgreSQL 14.4 on x86_64-apple-darwin21.5.0, compiled by Apple clang version 13.1.6 (clang-1316.0.21.2.5), 64-bit]
- pgx: `github.com/jackc/pgx/v4 v4.17.2`

Start with the pgxpool behavior exercised by Pool.Acquire and pool.Exec, then run the provided reproduction against PostgreSQL with exhausted connection slots. Determine whether SQLSTATE 53300 should be hidden while usable pool connections remain or surfaced when acquisition is canceled. Done requires a clearly decided behavior and regression coverage for the selected outcome.

Written by the indexing model from the issue text.