Errors from field resolvers in subscriptions are ignored

### What happened?

It seems like errors (and panics) in a field resolver are ignored if the type is being returned as part of a subscription. Instead, it simply returns the zero value for the field. This is the case when either returning the error from the resolver function or using `graphql.Errorf`.

### What did you expect?

That errors and panics occurring during resolution of fields resolved as part of a subscription are returned in the `errors` key of the response.

Using the below, a subscription of:

```graphql
subscription {
todos {
id
name
}
}
```

Returns the following every one second:

```json
{
"data": {
"todos": {
"id": "",
"name": null
}
}
}
```

In contrast:

```graphql
query {
todos {
id
name
}
}
```

Returns

```json
{
"errors": [
{
"message": "context error!",
"path": [
"todos",
0,
"name"
]
},
{
"message": "error!",
"path": [
"todos",
0,
"name"
]
}
],
"data": {
"todos": [
{
"id": "",
"name": null
}
]
}
}
```

### Minimal graphql.schema and models to reproduce

graph/schema.graphqls

```graphql
directive @goField(
forceResolver: Boolean
name: String
) on INPUT_FIELD_DEFINITION | FIELD_DEFINITION

type Subscription {
todos: Todo!
}

type Query {
todos: [Todo!]!
}

type Todo {
id: ID!
name: String @goField(forceResolver: true)
}
```

graph/schema.resolvers.go

```go
package graph

// This file will be automatically regenerated based on the schema, any resolver implementations
// will be copied through when generating and any unknown code will be moved to the end.

import (
"context"
"errors"
"time"
"tmp/graph/generated"
"tmp/graph/model"

"github.com/99designs/gqlgen/graphql"
)

func (r *queryResolver) Todos(ctx context.Context) ([]*model.Todo, error) {
return []*model.Todo{
{},
}, nil
}

func (r *subscriptionResolver) Todos(ctx context.Context) (<-chan *model.Todo, error) {
ch := make(chan *model.Todo)
go func() {
defer close(ch)
for {
ch <- &model.Todo{}
time.Sleep(time.Second)
}
}()
return ch, nil
}

func (r *todoResolver) Name(ctx context.Context, obj *model.Todo) (*string, error) {
//panic("panic!") // also ignored
graphql.AddErrorf(ctx, "context error!")
return nil, errors.New("error!")
}

// Query returns generated.QueryResolver implementation.
func (r *Resolver) Query() generated.QueryResolver { return &queryResolver{r} }

// Subscription returns generated.SubscriptionResolver implementation.
func (r *Resolver) Subscription() generated.SubscriptionResolver { return &subscriptionResolver{r} }

// Todo returns generated.TodoResolver implementation.
func (r *Resolver) Todo() generated.TodoResolver { return &todoResolver{r} }

type queryResolver struct{ *Resolver }
type subscriptionResolver struct{ *Resolver }
type todoResolver struct{ *Resolver }
```

graph/model/models_gen.go
```go
// Code generated by github.com/99designs/gqlgen, DO NOT EDIT.

package model

type Todo struct {
ID string `json:"id"`
Name *string `json:"name"`
}
```

### versions
- gqlgen version: `v0.11.3`
- `go version go1.14.2 linux/amd64`

```
module tmp

go 1.14

require (
github.com/99designs/gqlgen v0.11.3
github.com/cpuguy83/go-md2man/v2 v2.0.0 // indirect
github.com/gorilla/websocket v1.4.2 // indirect
github.com/hashicorp/golang-lru v0.5.4 // indirect
github.com/matryer/moq v0.0.0-20200607124540-4638a53893e6 // indirect
github.com/mitchellh/mapstructure v1.3.2 // indirect
github.com/pkg/errors v0.9.1 // indirect
github.com/urfave/cli/v2 v2.2.0 // indirect
github.com/vektah/dataloaden v0.3.0 // indirect
github.com/vektah/gqlparser/v2 v2.0.1
golang.org/x/mod v0.3.0 // indirect
golang.org/x/tools v0.0.0-20200611191743-782c6b3cc724 // indirect
gopkg.in/yaml.v2 v2.3.0 // indirect
)
```