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

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 <rzarzyns@redhat.com>