# 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
}
```

'Principal': {'Service': 's3.amazonaws.com'}
                }
            ]
        }
        policy_document = {
            'Version': '2012-10-17',
            'Statement': [
                {
                    'Action': [
                        's3:GetReplicationConfiguration',
                        's3:GetObjectVersion',
                        's3:ListBucket'
                    ],
                    'Effect': 'Allow',
                    'Resource': [
                        'arn:aws:s3:::bucket_name'
                    ]
                }
            ]
        }
        self.iam_client.create_role(
            RoleName=role_name,
            AssumeRolePolicyDocument=json.dumps(assume_policy_document)
        )
        policy_arn = self.iam_client.create_policy(
            PolicyName=policy_name,
            Path='/',
            PolicyDocument=json.dumps(policy_document)
        )['Policy']['Arn']
        policy_arns.append(policy_arn)