utils/exceptions: add maybe_rethrow_exception()
utils/exceptions: add maybe_rethrow_exception()

Helps with the common coroutine exception-handling idiom:

    std::exception_ptr ex;
    try {
        ...
    } catch (...) {
        ex = std::current_exception();
    }

    // release resource(s)
    maybe_rethrow_exception(std::move(ex));

    return result;

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla