Instead of raising std::runtime_error("Dangling queue_reader_handle_v2")
unconditionally. push() already raises _ex if set, best to be
consistent.
Unconditionally raising std::runtime_error can cause an error to be
logged, when aborting an operation involving a queue reader.
Although the original exception passed to
queue_reader_handle_v2::abort() is most likely handled by higher level
code (not logged), the generic std::runtime_error raised is not and
therefore is logged.
Fixes: #23550Closesscylladb/scylladb#23554
07510c07a0