### 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_
src_key = "key-to-be-copied"
        client.put_(Bucket=bucket_name, Key=src_key, Body=b"test-data")
        with pytest.raises(ClientError) as e:
            client.copy_(
                Bucket=bucket_name,
                Key="copy-test-sse-validate-kms-key-no-check",
                CopySource={"Bucket": bucket_name, "Key": src_key},
                ServerSideEncryption="aws:kms",
                SSEKMSKeyId="fake-key-id",
            )
        snapshot.match("copy-obj-wrong-kms-key", e.value.response)
    @pytest.mark.skipif(condition=TEST_S3_IMAGE, reason="KMS not enabled in S3 image")
    @markers.aws.validated
    @markers.snapshot.skip_snapshot_verify(
        paths=[
            "$..ETag",  # the 
        ]
    )
    def test_s3_sse_validate_kms_key_state(
        self, s3_bucket, kms_create_key, monkeypatch, snapshot, aws_client
    ):
        snapshot.add_transformer(snapshot.transform.key_value("Description"))
        data = b"test-sse"
        # crea
        kms_key = kms_create_key()
        # snap
        snapshot.match("create-kms-key", kms_key)
        key_name = "put--with-sse"
        put__with_sse = aws_client.s3.put_(
            Bucket=s3_bucket,