### 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_
def test_s3_get__preconditions(self, s3_bucket, snapshot, aws_client, method):
        snapshot.add_transformer(snapshot.transform.s3_api())
        _key = "test-"
        put_ = aws_client.s3.put_(
            Bucket=s3_bucket,
            Key=_key,
            Body=b"data",
        )
        head_ = aws_client.s3.head_(Bucket=s3_bucket, Key=_key)
        client_method = getattr(aws_client.s3, method)
        # wa
        # S3 
        # It 
        time.sleep(3)
        # we
        now = datetime.datetime.now().astimezone(tz=ZoneInfo("GMT"))
        wrong_unmodified_since = now - datetime.timedelta(days=1)
        with pytest.raises(ClientError) as e:
            client_method(
                Bucket=s3_bucket,
                Key=_key,
                IfModifiedSince=now,
                IfUnmodifiedSince=wrong_unmodified_since,
                IfMatch="etag123",
                IfNoneMatch=put_["ETag"],
            )
        snapshot.match("precondition-if-match", e.value.response)
        with pytest.raises(ClientError) as e:
            client_method(
                Bucket=s3_bucket,