### 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_
data = "1234567890 " * 100
        
        upload_file_ = BytesIO()
        mtime = 1676569620  # hard
        with gzip.GzipFile(fileobj=upload_file_, mode="w", mtime=mtime) as filestream:
            filestream.write(data.encode("utf-8"))
        # Up
        response = aws_client.s3.put_(
            Bucket=s3_bucket,
            Key="test.gz",
            ContentEncoding="gzip",
            Body=upload_file_.getvalue(),
        )
        snapshot.match("put-", response)
        # Down
        downloaded_ = aws_client.s3.get_(Bucket=s3_bucket, Key="test.gz")
        snapshot.match("get-", downloaded_)
        download_file_ = BytesIO(downloaded_["Body"].read())
        with gzip.GzipFile(fileobj=download_file_, mode="rb") as filestream:
            downloaded_data = filestream.read().decode("utf-8")
        assert downloaded_data == data
    @markers.aws.validated
    @markers.snapshot.skip_snapshot_verify(
        condition=is_v2_provider,
        paths=["$..ServerSideEncryption"],
    )
    def test_multipart_overwrite_key(self, s3_bucket, s3_multipart_upload, snapshot, aws_client):