cashew: libcurl_http_fetcher improvements

This CL enhances cashew's libcurl http fetcher to address some issues
that I noticed while working on issue 16250:
- Log more specific error information in the event of a request failure
- Reduce tcp and http timeouts to 30 seconds
- Reduce log spam during a lengthy request by not generating a log msg
  each time that we enter the TimeoutCallback (once per second).

BUG=None
TEST=Ran unit tests and performed manual testing on device

Change-Id: Iae7b72661b4723bb3910d60fd7e63292088e3857
Reviewed-on: http://gerrit.chromium.org/gerrit/7187
Reviewed-by: Jason Glasgow <jglasgow@chromium.org>
Tested-by: Vince Laviano <vlaviano@chromium.org>