AllowExplain exiting postgraphile run as library
<!-- Before opening an issue: if you use `npm` try running `yarn` instead; if
      you run `yarn`, please try running `npx yarn-deduplicate && yarn`. -->

### Summary

if allowExplain is enabled and explain is enabled in graphiql website then postgraphile error with exit 1.
<!-- A brief summary of the problem. -->

### Steps to reproduce

Create database as:
```
drop schema if exists app_public CASCADE;
drop role IF EXISTS app_role;
create role app_role;
create schema if not exists app_public;
CREATE EXTENSION IF NOT EXISTS citext WITH SCHEMA app_public;
GRANT usage ON SCHEMA app_public TO app_role;
ALTER DEFAULT privileges REVOKE EXECUTE ON functions FROM public;
REVOKE ALL ON DATABASE app_public FROM PUBLIC;
REVOKE ALL ON SCHEMA public FROM PUBLIC;

create table if not exists app_public.foo (
 id serial primary key,
 title text not null
);

GRANT SELECT ON TABLE app_public.foo TO app_role;

insert into app_public.foo (title) values ('one'), ('two'), ('three');
create or replace function app_public.foo_test_ha (o app_public.foo) RETURNS text AS $$ SELECT 'text' $$ LANGUAGE sql STABLE;
```

create postgraphile as library as:
```
import express from "express";
import { postgraphile } from "postgraphile";
const postgraphileOptions = {
  pgSettings: () => ({ role: "app_role" }),
  graphiql: true,
  enhanceGraphiql: true,
  allowExplain: true
}
const postgraphileMiddleware = postgraphile("postgres://postgres:postgres@localhost/app_public", "app_public", postgraphileOptions)
const app = express();
app.use(postgraphileMiddleware);
app.listen(port, () => {
  console.log(`Pricing service listening on http://localhost:${port}/graphiql`);
});
```

Run db.
Run `yarn server`

Run query with explain disabled:
```
query MyQuery {
  allFoos {
    nodes {
      title
      testHa
    }
  }
}
```

now turn on explain and postgraphile will crash.

<!-- All bug reports should include detailed steps to reproduce. -->

### Expected results

Same as with explain disabled - message with permission denied.

<!-- Outline what you expected to see -->

### Actual results

<!-- Outline the actual results -->

### Additional context

Postgresql postgres:13-alpine
Postgraphile: "postgraphile": "^4.10.0"
Yarn: v1.22.10
node: v15.4.0

<!-- Please provide additional context such as environment versions (including
      operating system, Node, PostgreSQL, Redis, etc. as relevant), software
      versions (from `yarn.lock`) or similar, plugins in use, database schema
      if relevant, bundler (if any). -->

### Possible Solution

<!--- Optionally suggest a fix or reason for the bug. -->
