Improve error reporting for missing fields

See #13, a query against a missing field results in runtime exception:

```
TypeError: Cannot read properties of undefined (reading 'fieldsByTypeName')
    at resolve_tree (/usr/app/node_modules/objection-graphql-resolver/src/resolver/graph.ts:62:30)
```

which is rather cryptic. The message should be improved and suggest user what to do.