### 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_
request: CopyObjectRequest, last_modified: datetime.datetime, etag: ETag
) -> Optional[str]:
    """
    Validate if the source object LastModified and ETag matches a precondition, and if it does, return the failed
    precondition
    # see https://docs.aws.amazon.com/AmazonS3/latest/API/API_CopyObject.html
    :param request: the CopyObjectRequest
    :param last_modified: source object LastModified
    :param etag: source object ETag
    :return str: the failed precondition to raise
    """
    if (cs_if_match := request.get("CopySourceIfMatch")) and etag.strip('"') != cs_if_match.strip(
        '"'
    ):
        return "x-amz-copy-source-If-Match"
    elif (
        cs_if_unmodified_since := request.get("CopySourceIfUnmodifiedSince")
    ) and last_modified > cs_if_unmodified_since:
        return "x-amz-copy-source-If-Unmodified-Since"
    elif (cs_if_none_match := request.get("CopySourceIfNoneMatch")) and etag.strip(
        '"'
    ) == cs_if_none_match.strip('"'):
        return "x-amz-copy-source-If-None-Match"
    elif (
        cs_if_modified_since := request.get("CopySourceIfModifiedSince")
    ) and last_modified < cs_if_modified_since < datetime.datetime.now(tz=_gmt_zone_info):
        return "x-amz-copy-source-If-Modified-Since"
def validate_failed_precondition(