### 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_
snapshot.match("obj-success", get_obj_all_positive)
    @markers.aws.validated
    def test_s3_multipart_upload_acls(
        self, s3_bucket, allow_bucket_acl, s3_multipart_upload, snapshot, aws_client
    ):
        # ht
        # > 
        # > 
        # > 
        snapshot.add_transformer(
            [
                snapshot.transform.key_value("DisplayName"),
                snapshot.transform.key_value("ID", value_replacement="owner-id"),
            ]
        )
        aws_client.s3.put_bucket_acl(Bucket=s3_bucket, ACL="public-read")
        response = aws_client.s3.get_bucket_acl(Bucket=s3_bucket)
        snapshot.match("bucket-acl", response)
        def check_permissions(key):
            acl_response = aws_client.s3.get__acl(Bucket=s3_bucket, Key=key)
            snapshot.match(f"permission-{key}", acl_response)
        # perf
        aws_client.s3.put_(Bucket=s3_bucket, Key="acl-key0", Body="something")
        check_permissions("acl-key0")
        s3_multipart_upload(bucket=s3_bucket, key="acl-key1")
        check_permissions("acl-key1")
        s3_multipart_upload(bucket=s3_bucket, key="acl-key2", acl="public-read-write")
        check_permissions("acl-key2")
    @markers.aws.validated
    def test_s3_bucket_acl(self, s3_bucket, allow_bucket_acl, snapshot, aws_client):