### 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_
snapshot.transform.key_value("UploadId"),
                snapshot.transform.key_value("Location"),
            ]
        )
        key_name = "test-sse-field-multipart"
        data = b"test-sse"
        key_id = kms_create_key()["KeyId"]
        # if y
        # othe
        # but 
        s3_multipart_upload_with_snapshot(
            bucket=s3_bucket,
            key=key_name,
            data=data,
            snapshot_prefix="multi-sse",
            BucketKeyEnabled=True,
            SSEKMSKeyId=key_id,
            ServerSideEncryption="aws:kms",
        )
        response = aws_client.s3.get_(Bucket=s3_bucket, Key=key_name)
        snapshot.match("get-obj", response)
    @pytest.mark.skipif(condition=TEST_S3_IMAGE, reason="KMS not enabled in S3 image")
    @markers.aws.validated
    # there is currently no server side encryption is place in LS, ETag will be different
    @markers.snapshot.skip_snapshot_verify(paths=["$..ETag"])
    @markers.snapshot.skip_snapshot_verify(
        condition=is_v2_provider,
        paths=["$..ServerSideEncryption"],
    )
    def test_s3_sse_bucket_key_default(