Aborting a stream plan is used in deinitialization code ran in noexcept environment, so it should be noexcept itself. Tested on a not-merged-yet Seastar patch with hardened noexcept checks for abort_source. Message-Id: <6eada033bb394d725b83a7e0f92381cb792ef6a1.1596446857.git.sarna@scylladb.com>