Including http_error as attribute in exception

It looks to me like the current implementation of `ArtifactoryException` currently only includes the http error code as part of the string message (e.g. `dohq_artifactory.exception.ArtifactoryException: 502 Server Error: Bad Gateway for url: https://myserver.com/artifactory/api/storage/my-repo/path/4242/directory`
Unfortunately our infrastructure is sometimes unstable so we need to retry some operations in case of some specific errors (e.g. 429).

We prefer not to parse the exception message string, as there are some numbers (IDs) in the URL and we would create a dependency to the format of your exception message.

Instead, we think that including an optional (i.e. None on initialization) `http_error_code` would greatly help us. That is anyway what the `HTTPError` of `requests` includes: https://github.com/psf/requests/blob/main/src/requests/exceptions.py#L22