# Type of request: This is a ...

[X] bug report

# Detailed description
`EntityAlreadyExistsException` is not thrown correctly when creating IAM objects that are already present. `AmazonIdentityManagementException` with a null message is thrown instead

## Expected behavior
Localstack should throw `EntityAlreadyExistsException` with a populated message (not null)

## Actual behavior
```
com.amazonaws.services.identitymanagement.model.AmazonIdentityManagementException: null (Service: AmazonIdentityManagement; Status Code: 409; Error Code: 409 Conflict; Request ID: null)
```

# Steps to reproduce
- create an IAM role
- try to re-create it, catch `EntityAlreadyExistsException` but `AmazonIdentityManagementException` with null message is thrown instead

## Command used to start LocalStack
docker-compose up with `0.10.9`

## Client code (AWS SDK code snippet, or sequence of "awslocal" commands)
```
try {
    localStackIAMClient.createRole(createRoleRequest);
    localStackIAMClient.createRole(createRoleRequest);
} catch (EntityAlreadyExistsException e) {
    // AmazonIdentityManagementException with null is thrown instead
}
```

test_policy_document = {
            'Version': '2012-10-17',
            'Statement': {
                'Effect': 'Allow',
                'Action': 's3:ListBucket',
                'Resource': 'arn:aws:s3:::example_bucket'
            }
        }
        test_user_name = 'test-user'
        self.iam_client.create_user(UserName=test_user_name)
        response = self.iam_client.create_policy(
            PolicyName='test-policy',
            PolicyDocument=json.dumps(test_policy_document)
        )
        test_policy_arn = response['Policy']['Arn']
        self.assertIn(TEST_AWS_ACCOUNT_ID, test_policy_arn)
        self.iam_client.attach_user_policy(UserName=test_user_name, PolicyArn=test_policy_arn)
        attached_user_policies = self.iam_client.list_attached_user_policies(UserName=test_user_name)
        self.assertEqual(len(attached_user_policies['AttachedPolicies']), 1)
        self.assertEqual(attached_user_policies['AttachedPolicies'][0]['PolicyArn'], test_policy_arn)
        
        self.iam_client.detach_user_policy(
            UserName=test_user_name,
            PolicyArn=test_policy_arn
        )
        self.iam_client.delete_policy(
            PolicyArn=test_policy_arn
        )