Fix deprecation warning in std.concurrency.

Replace false with Fiber.Rethrow.no to remove the warning.
use a Flag!"throwOnError" for Curl.perform
allow to specify throwOnError for high-level wrappers

- in HTTP, FTP, SMTP perform
Merge pull request #2760 from redstar/concurrency

Fix deprecation warning in std.concurrency.
Merge pull request #2750 from MartinNowak/curl2

ThrowOnError flag for curl