status code 204 should be returned if file is not found in disk

[cnlangzi]

check io err no such file or directory. status code 204 should be returned like we did on 0chain/zboxcli#99

[princeparmar]

@cnlangzi Already done
Gosdk : 0chain/gosdk#396
zBox : 0chain/zboxcli#181
Blobber : #551

[cnlangzi]

a new bug is introduced by 204 on #385 (comment)

[cnlangzi]

waiting for #592 to merge first.