# 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
}
```

RoleName=role_name
        )
    def test_create_user_with_tags(self):
        user_name = 'user-role-{}'.format(short_uid())
        rs = self.iam_client.create_user(
            UserName=user_name,
            Tags=[
                {'Key': 'env', 'Value': 'production'}
            ]
        )
        self.assertIn('Tags', rs['User'])
        self.assertEqual(rs['User']['Tags'][0]['Key'], 'env')
        rs = self.iam_client.get_user(
            UserName=user_name
        )
        self.assertIn('Tags', rs['User'])
        self.assertEqual(rs['User']['Tags'][0]['Value'], 'production')
        
        self.iam_client.delete_user(
            UserName=user_name
        )
    def test_attach_detach_role_policy(self):
        role_name = 's3-role-{}'.format(short_uid())
        policy_name = 's3-role-policy-{}'.format(short_uid())
        policy_arns = [p['Arn'] for p in ADDITIONAL_MANAGED_POLICIES.values()]
        assume_policy_document = {
            'Version': '2012-10-17',
            'Statement': [
                {
                    'Action': 'sts:AssumeRole',