### 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_
response = aws_client.s3.get_(Bucket=s3_bucket, Key="mykey")
        snapshot.match("get-", response)
        response = aws_client.s3.copy_(
            Bucket=s3_bucket,
            CopySource=f"{s3_bucket}/mykey",
            Key="copiedkey",
            BucketKeyEnabled=True,
            SSEKMSKeyId=key_id,
            ServerSideEncryption="aws:kms",
        )
        snapshot.match("copy-", response)
        response = aws_client.s3.get_(Bucket=s3_bucket, Key="copiedkey")
        snapshot.match("get-copied-", response)
    @markers.aws.validated
    @markers.snapshot.skip_snapshot_verify(paths=["$..AccessPointAlias"])
    def test_region_header_exists(self, s3_create_bucket, snapshot, aws_client):
        snapshot.add_transformer(snapshot.transform.s3_api())
        bucket_name = s3_create_bucket(
            CreateBucketConfiguration={"LocationConstraint": "eu-west-1"},
        )
        response = aws_client.s3.head_bucket(Bucket=bucket_name)
        assert response["ResponseMetadata"]["HTTPHeaders"]["x-amz-bucket-region"] == "eu-west-1"
        snapshot.match("head_bucket", response)
        response = aws_client.s3.list_s_v2(Bucket=bucket_name)
        assert response["ResponseMetadata"]["HTTPHeaders"]["x-amz-bucket-region"] == "eu-west-1"
        snapshot.match("list_s_v2", response)
    @markers.aws.validated
    
    @markers.snapshot.skip_snapshot_verify(paths=["$..Prefix", "$..list-buckets.Buckets"])
    def test_delete_bucket_with_content(self, s3_bucket, s3_empty_bucket, snapshot, aws_client):