chore: add missing error checks

[thevilledev]

Overview

• Add missing error check to main for parsing CLI flags.
• Add missing error checks to tests for I/O related calls.

If flag.Parse() fails now the error is hidden from the user. Adding the missing checks to tests just improve code quality.

Design of Change

Noticed an issue with CLI flag parsing. Fixed it first. Checked with errcheck linter if the codebase has other missing error checks.

Related Issues/Pull Requests

None

Contributor Checklist

[-] Add relevant docs to upstream Vault repository, or sufficient reasoning why docs won’t be added yet

None needed

[-] Add output for any tests not ran in CI to the PR description (eg, acceptance tests)

None needed

[x] Backwards compatible