The original idea is to keep the code consistent with getFileLocation() in Master side, so that the user receives the similar Exception prompt here when facing the DIR case.
```
      Inode inode = mInodes.get(fileId);
      if (inode == null || inode.isDirectory()) {
        throw new FileDoesNotExistException("FileId " + fileId + " does not exist.");
      }
```
See also those exceptions thrown out from cacheBlock() in Master side.
```
     if (inode.isDirectory()) {
        throw new FileDoesNotExistException("File " + fileId + " is a folder.");
      }
```
If this doesn't matter, we can change it.