### 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.upload_part(
                Bucket=s3_bucket,
                Key=key_name,
                Body=upload_file_,
                PartNumber=1,
                UploadId=upload_id,
            )
        snapshot.match("upload-part-no-checksum-exc", e.value.response)
    @markers.aws.validated
    @pytest.mark.skipif(
        condition=LEGACY_V2_S3_PROVIDER,
        reason="Behaviour not implemented yet: https://github.com/localstack/localstack/issues/6882",
    )
    @pytest.mark.skipif(condition=TEST_S3_IMAGE, reason="KMS not enabled in S3 image")
    # there is currently no server side encryption is place in LS, ETag will be different
    @markers.snapshot.skip_snapshot_verify(paths=["$..ETag"])
    def test_s3_multipart_upload_sse(
        self,
        aws_client,
        s3_bucket,
        s3_multipart_upload_with_snapshot,
        kms_create_key,
        snapshot,
    ):
        snapshot.add_transformer(
            [
                snapshot.transform.resource_name("SSEKMSKeyId"),
                snapshot.transform.key_value(
                    "Bucket", reference_replacement=False, value_replacement="<bucket>"
                ),