[C++][Pistache] Catch HttpError from user-provided handler

Created by: muttleyxd

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@ravinikam (2017/07) @stkrwork (2017/07) @etherealjoy (2018/02) @MartinDelille (2018/03)

I tried to use Pistache::Http::serveFile, and it works, but when the file does not exist, then it will return 500, instead of 404. That's because it throws Pistache::Http::HttpError with 404 code, but it's being caught by catch (std::exception &) and 500 is returned afterwards, despite original exception having more information.

void MyHandlers::getFile(std::string const& path, Pistache::Http::ResponseWriter& response)
{
    Pistache::Http::serveFile(response, path.get());
}

Also this will allow user to use exceptions to signal errors to REST API caller:

void MyHandlers::someEndpoint(Pistache::Http::ResponseWriter& response)
{
    if (some_precondition_failed())
        throw Pistache::Http::HttpError(500, "precondition has failed");
    response.send(Pistache::Http::Code::Ok, "Success");
}