[Bug] Incorrect formatting for digest in SOCI index remove

[austinvazquez]

Description

The error message for SOCI index remove command when the index digest is not found can be better formatted to communicate the error.

[soci-snapshotter]$ sudo soci index ls
DIGEST    SIZE    IMAGE REF    PLATFORM    MEDIA TYPE    CREATED    
[soci-snapshotter]$ sudo soci index rm sha256:doesnotexist
soci: the index of the digest [115 104 97 50 53 54 58 100 111 101 115 110 111 116 101 120 105 115 116] doesn't exist
[soci-snapshotter]$ 

Steps to reproduce the bug

1. sudo soci index rm sha256:doesnotexist

Describe the results you expected

Expected the message to be:

soci: the index of the digest sha256:doesnotexist doesn't exist

Host information

1. OS: Amazon Linux 2023
2. Snapshotter Version: soci version 2cb31a4 2cb31a4
3. Containerd Version: containerd github.com/containerd/containerd v1.6.31-21-g8cf85ba56 8cf85ba5657e5a87c0a700f1395af20b33aa4990

Any additional context or information about the bug

No response