Improve error handling of update-format-crau

[dongsupark]

• Currently parse_signature_data and verify_sig_pubkey return Option instead of Result. That was written for simplicity. But we are seeing more cases where clearer error handling with Result is needed.
  /cc @wrl

  • Change return types to Result in signature verification #30

• the dummy signature error is printed too prominently, we skip over it but it looks like the program crashed if you don't pay attention to the last line

Parsed and verified signature data from file "/var/tmp/outdir/.unverified/oem-ami.gz"

which indicates that it was successful.
(suggested by @pothos)

[pothos]

The comment is here: #23 (comment)

[dongsupark]

The 1st issue was done by #30

[pothos]

The anyhow PR removed the printing of the signature error, we can close this.