http: make curl error category more appropriate

Previous error category, boost::system::system_category mapped into POSIX
error codes, which was unsuitable for curl.

Change-Id: I8978047fba09225ec496da60fb41c6b5042591de
Signed-off-by: Bartlomiej Wolowiec <bartlomiej.wolowiec@codilime.com>
Signed-off-by: Mateusz Zalega <mateusz.zalega@codilime.com>
Merge "http: make curl error category more appropriate"