This eliminates some overhead, because the compiler doesn't need to
consider these functions throwing.

If an error has already been set by f.directory->storage->GetInfo(),
don't set it again.

It was used in a wrong way, which did not deal with errors
consistently.  And if that's wrong, there is no need for FindStorage()
at all - let's remove it and the confusion around it.

"override" implies "virtual".

This is the backend for the upcoming "mount" command.