Fix bug #147530 by enabling more robust error code definitions.

* bzrlib/transport/http/_pycurl.py:
(_get_pycurl_errcode): Wraps error code definition ensuring better
backward/forward compatibility with curl.
(PyCurlTransport._curl_perform): Use our new error code symbols.

* bzrlib/transport/http/_pycurl_errors.py:
Deleted. The design couldn't cope with being compatible with
several pycurl versions.