Add bucket to be part of object struct, reply back with BucketNotFound

for erroneous Object Put requests.

Other minor cleanups, to follow

  - https://github.com/golang/go/wiki/CodeReviewComments

pkg/storage/storage.go

@@ -40,6 +40,7 @@ type BucketMetadata struct {
 }
 
 type ObjectMetadata struct {
+	Bucket  string
 	Key     string
 	Created time.Time
 	Size    int