More complete fix.

* bzrlib/transport/http/_urllib2_wrappers.py:
(AbstractHTTPHandler.retry_or_raise): ECONNRESET should be
translated into ConnectionReset. It's pretty rare that it can
occur during the request sending, but it has been observed during
selftest runs.

* bzrlib/transport/http/_pycurl.py:
(PyCurlTransport._curl_perform): Turns CURLE_RECV_ERROR into a
ConnectionReset instead of a ConnectionError or some callers won't
catch it properly.

* bzrlib/transport/http/__init__.py:
(SmartClientHTTPMedium.send_http_smart_request): Catch
ConnectionReset and turn into a SmartProtocolError like for
InvalidHttpResponse.

* bzrlib/tests/test_http.py:
(TestWallServer.test_http_has, TestWallServer.test_http_get):
ConnectionReset can be raised too.