Improve the error message when trying to use OPFS outside of a worker

# Description
I'm running into an issue when trying to create a new database using the OPFS AHP file system. This snippet
```typescript
import { PGlite } from "@electric-sql/pglite";

const client = await PGlite.create({
dataDir: "opfs-ahp://electric.db",
relaxedDurability: true,
});
```

is producing this error


I have a minimal-repro repo here that has the issue: https://github.com/TheAndrewJackson/opfs-ahp_bug_repro