crimson/ertr: let ErrVisitorT return plain value if ValueFuncT returns seastar::future

The following corner case was unsupported till now:

```cpp
  return os->omap_get_values(
    ch, hoid, keys
  ).safe_then([] (FuturizedStore::omap_values_t&& vals) {
    return 0;
  }, FuturizedStore::read_errorator::all_same_way([] (auto&) {
    return -1;
  })).get();
```
Signed-off-by: Radoslaw Zarzynski <[email protected]>

Author: rzarzynski

src/crimson/common/errorator.h

@@ -335,6 +335,13 @@ class maybe_handle_error_t {
         // to throwing an exception by the handler.
         std::invoke(std::forward<ErrorVisitorT>(errfunc),
                     ErrorT::error_t::from_exception_ptr(std::move(ep)));
+      } else if constexpr (seastar::Future<decltype(result)>) {
+        // result is seastar::future but return_t is e.g. int. If so,
+        // the else clause cannot be used as seastar::future lacks
+        // errorator_type member.
+        result = seastar::make_ready_future<return_t>(
+          std::invoke(std::forward<ErrorVisitorT>(errfunc),
+                      ErrorT::error_t::from_exception_ptr(std::move(ep))));
       } else {
         result = FuturatorT::type::errorator_type::template make_ready_future<return_t>(
           std::invoke(std::forward<ErrorVisitorT>(errfunc),