### Is there an existing issue for this?

- [X] I have searched the existing issues

### Current Behavior

- Localstack fails with HTTP 500, error code "InternalError", message "not enough values to unpack (expected 2, got 1)" when `CopySource` is an invalid string for `copy_object` API.

### Expected Behavior

- For the same API `copy_object`, the cloud fails with HTTP 400, error code "InvalidArgument".
- Localstack should handle invalid `CopySource` i.e., it is a dictionary with one value or an invalid string not adhering to "{bucket}/{key}" format.

### How are you starting LocalStack?

With a `docker run` command

### Steps To Reproduce

#### How are you starting localstack (e.g., `bin/localstack` command, arguments, or `docker-compose.yml`)

    docker run localstack/localstack

#### Client commands (e.g., AWS SDK code snippet, or sequence of "awslocal" commands)

    botoclient.copy_object(CopySource="Invalid_random_string", Bucket="test_bucket", Key=key) 


 test_bucket is a valid bucket.  

### Environment

```markdown
- OS: Ubuntu 22.04.2 LTS
- LocalStack: 2.1.0
- boto3: 1.28.6
```


### Anything else?

_No response_
@markers.aws.validated
    def test_presigned_url_with_different_user_credentials(
        self,
        aws_client,
        s3_create_bucket_with_client,
        create_role_with_policy,
        account_id,
        wait_and_assume_role,
        patch_s3_skip_signature_validation_false,
    ):
        bucket_name = f"bucket-{short_uid()}"
        key_name = "key"
        actions = [
            "s3:CreateBucket",
            "s3:Pu",
            "s3:Ge",
            "s3:DeleteBucket",
            "s3:Delete",
        ]
        assume_policy_doc = {
            "Version": "2012-10-17",
            "Statement": [
                {
                    "Action": "sts:AssumeRole",
                    "Principal": {"AWS": account_id},
                    "Effect": "Allow",
                }
            ],
        }
        assume_policy_doc = json.dumps(assume_policy_doc)