### 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_
key = "test-checksum-retrieval"
        body = b"test-checksum"
        kwargs = {}
        if algorithm:
            kwargs["ChecksumAlgorithm"] = algorithm
        put_ = aws_client.s3.put_(Bucket=s3_bucket, Key=key, Body=body, **kwargs)
        snapshot.match("put-", put_)
        get_ = aws_client.s3.get_(Bucket=s3_bucket, Key=key)
        snapshot.match("get-", get_)
        get__with_checksum = aws_client.s3.get_(
            Bucket=s3_bucket, Key=key, ChecksumMode="ENABLED"
        )
        snapshot.match("get--with-checksum", get__with_checksum)
        # te
        head__with_checksum = aws_client.s3.get_(
            Bucket=s3_bucket, Key=key, ChecksumMode="enabled"
        )
        snapshot.match("head--with-checksum", head__with_checksum)
        _attrs = aws_client.s3.get__attributes(
            Bucket=s3_bucket,
            Key=key,
            Attributes=["Checksum"],
        )
        snapshot.match("get--attrs", _attrs)
    @markers.aws.validated
    @markers.snapshot.skip_snapshot_verify(
        condition=is_v2_provider,
        paths=["$..ServerSideEncryption"],
    )
    def test_s3_checksum_with_content_encoding(self, s3_bucket, snapshot, aws_client):