### 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_
# ge
        _attrs = aws_client.s3.get__attributes(
            Bucket=bucket,
            Key=key,
            Attributes=["ETag", "Checksum"],
        )
        snapshot.match("get--attrs-generated", _attrs)
        # Te
        params.pop(f"Checksum{algorithm}")
        response = aws_client.s3.put_(**params)
        snapshot.match("put--autogenerated", response)
        assert response["ResponseMetadata"]["HTTPStatusCode"] == 200
        # ge
        _attrs = aws_client.s3.get__attributes(
            Bucket=bucket,
            Key=key,
            Attributes=["ETag", "Checksum"],
        )
        snapshot.match("get--attrs-auto-generated", _attrs)
        get__with_checksum = aws_client.s3.head_(
            Bucket=bucket, Key=key, ChecksumMode="ENABLED"
        )
        snapshot.match("head--with-checksum", get__with_checksum)
    @markers.aws.validated
    @pytest.mark.parametrize("algorithm", ["CRC32", "CRC32C", "SHA1", "SHA256", None])
    @markers.snapshot.skip_snapshot_verify(
        condition=is_v2_provider,
        paths=["$..ServerSideEncryption"],
    )
    def test_s3_get__checksum(self, s3_bucket, snapshot, algorithm, aws_client):