xl: Fix isObject() to consider not found disks (#8411)
xl: Fix isObject() to consider not found disks (#8411)

xl.isObject() returns 'nil' for not found disks when
calculating the existance of xl.json for a given object,
which what StatFile() is also doing (setting nil) if
xl.json exists.

This commit avoids this confusion by setting errDiskNotFound
error when the storage disk is not found.
balamurugana/minio,aead/minio,aead/minio,harshavardhana/minio,krisis/minio,harshavardhana/minio,krisis/minio,fwessels/minio,fwessels/minio,harshavardhana/minio,aead/minio,aead/minio,donatello/minio,aead/minio,harshavardhana/minio,aead/minio,minio/minio,fwessels/minio,harshavardhana/minio,balamurugana/minio,NitishT/minio,harshavardhana/minio,NitishT/minio,harshavardhana/minio,vadmeste/minio,donatello/minio,aead/minio,vadmeste/minio,NitishT/minio,fwessels/minio,harshavardhana/minio,NitishT/minio,aead/minio,minio/minio