### 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-key"
        aws_client.s3.put_(Bucket=s3_bucket, Key=_key, Body="test-body")
        def assert_timestamp_is_iso8061_s3_format(_timestamp: str):
            # the 
            # 2023
            assert _timestamp.endswith(".000Z")
            assert len(_timestamp) == 24
            # asse
            parsed_ts = datetime.datetime.strptime(_timestamp, "%Y-%m-%dT%H:%M:%S.%fZ")
            assert parsed_ts.microsecond == 0
        s3_http_client = aws_http_client_factory("s3", signer_factory=SigV4Auth)
        list_buckets_endpoint = _endpoint_url()
        list_buckets_resp = s3_http_client.get(
            list_buckets_endpoint, headers={"x-amz-content-sha256": "UNSIGNED-PAYLOAD"}
        )
        list_buckets_dict = xmltodict.parse(list_buckets_resp.content)
        buckets = list_buckets_dict["ListAllMyBucketsResult"]["Buckets"]["Bucket"]
        # beca
        if isinstance(buckets, list):
            bucket = buckets[0]
        else:
            bucket = buckets
        bucket_timestamp: str = bucket["CreationDate"]
        assert_timestamp_is_iso8061_s3_format(bucket_timestamp)
        bucket_url = _bucket_url(s3_bucket)
        _url = f"{bucket_url}/{_key}"
        head_obj_resp = s3_http_client.head(
            _url, headers={"x-amz-content-sha256": "UNSIGNED-PAYLOAD"}
        )
        last_modified: str = head_obj_resp.headers["Last-Modified"]