Improve "Could not find type named" error by citing plugin name or index and location in gql schema

This error:

```
((DEV)) @my-ns/server: A serious error occurred when building the initial schema (attempt 9). We'll try again in 8100ms. Error details:
((DEV)) @my-ns/server: Error: Could not find type named 'boolean'.
```

Would better read as:

```
Could not find type named 'boolean' in plugin 2 'MyCustomPlugin' in 'Query.myQuery'
```

As discussed just now in [Discord](https://discordapp.com/channels/489127045289476126/498852330754801666).