### 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_
aws_client.s3.get_(Bucket=s3_bucket, Key=key, PartNumber=10)
        snapshot.match("part-doesnt-exist", e.value.response)
        with pytest.raises(ClientError) as e:
            aws_client.s3.get_(
                Bucket=s3_bucket,
                Key=key,
                PartNumber=2,
                Range="bytes=0-8",
            )
        snapshot.match("part-with-range", e.value.response)
        key_no_part = "key-no-part"
        aws_client.s3.put_(Bucket=s3_bucket, Key=key_no_part, Body="test-123")
        with pytest.raises(ClientError) as e:
            aws_client.s3.get_(Bucket=s3_bucket, Key=key_no_part, PartNumber=2)
        snapshot.match("part-no-multipart", e.value.response)
        get_obj_no_part = aws_client.s3.get_(Bucket=s3_bucket, Key=key_no_part, PartNumber=1)
        snapshot.match("get-obj-no-multipart", get_obj_no_part)
    @markers.aws.validated
    @markers.snapshot.skip_snapshot_verify(
        condition=is_v2_provider,
        paths=["$..ServerSideEncryption"],
    )
    def test_set_external_hostname(
        self, s3_bucket, allow_bucket_acl, s3_multipart_upload, monkeypatch, snapshot, aws_client
    ):
        snapshot.add_transformer(
            [
                snapshot.transform.key_value("Location"),
                snapshot.transform.key_value("Bucket"),
            ]