Error message references the wrong type when an input type is invalid.

## Description

When a field in an input type is missing the reported error message is wrong.

There is an input type with a nested type:

```graphql
type Mutation {
createItem(outside_input: OutsideInput!): String
}

input OutsideInput {
nested: NestedInput
}

input NestedInput {
name: String!
}
```

When the `name` is missing from the variables (note that it's marked as required, so it's an input error):

```json
{"outside_input": {"nested": {}}}
```

The error message incorrectly points to the outer type:

```
Variable 'outside_input' has coerced Null value for NonNull type 'String!'
```

Note that the `outside_input` is:
* not `Null`
* not `String!`

Instead, the `NestedInput.name` is `Null` that is expected to be `String!`.

## Reproduction

Deploy this stack:

```yaml
AWSTemplateFormatVersion: '2010-09-09'
Resources:
AppSyncAPI:
Type: AWS::AppSync::GraphQLApi
Properties:
Name: InputValidationTestAPI
AuthenticationType: API_KEY

GraphQLSchema:
Type: AWS::AppSync::GraphQLSchema
Properties:
ApiId: !GetAtt AppSyncAPI.ApiId
Definition: |
type Mutation {
createItem(outside_input: OutsideInput!): String
}

input OutsideInput {
nested: NestedInput
}

input NestedInput {
name: String!
}

type Query {
dummy: String
}

schema {
query: Query
mutation: Mutation
}

APIKey:
Type: AWS::AppSync::ApiKey
Properties:
ApiId: !GetAtt AppSyncAPI.ApiId

```

Send this query:

```graphql
mutation MyMutation($outside_input: OutsideInput = {}) {
createItem(outside_input: $outside_input)
}
```

With these variables:

```json
{"outside_input": {"nested": {}}}
```

Observe the result:

```json
{
"data": null,
"errors": [
{
"path": null,
"locations": [
{
"line": 1,
"column": 21,
"sourceName": null
}
],
"message": "Variable 'outside_input' has coerced Null value for NonNull type 'String!'"
}
]
}
```

Start by deploying the provided AWS AppSync CloudFormation stack and running the mutation with the supplied variables. Trace how nested GraphQL input coercion constructs the validation error; done means the message identifies NestedInput.name as null for the expected String! type instead of referring to outside_input.

Written by the indexing model from the issue text.