fix(unikontainers): propagate actual errors instead of masking behind ErrNotUnikernel

[safiya2610]

Description

Summary of Changes:
Updated GetUnikernelConfig error handling in pkg/unikontainers/unikontainers.go.

Instead of masking all configuration/decoding errors into ErrNotUnikernel, it now properly propagates the actual parsing/base64 decoding errors to the user. It only falls back to runc (via ErrNotUnikernel) specifically when os.ErrNotExist occurs (i.e., no configuration exists).

This prevents silent failures and makes debugging malformed configurations much easier.

Related issues

• Fixes Bug: Masked configuration/decoding errors cause silent fallback to runc and confusing failures #876

How was this tested?

Verified that malformed configurations correctly throw detailed parsing/decoding errors instead of failing silently and falling back to runc.

LLM usage

I used Google Gemini to help draft this pull request description.

Checklist

• I have read the contribution guide.
• The linter passes locally (make lint).
• The e2e tests of at least one tool pass locally (make test_ctr, make test_nerdctl, make test_docker, make test_crictl).
• If LLMs were used: I have read the llm policy.

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[Nachiket-Roy]

Don't overwrite the PR template

[safiya2610]

Don't overwrite the PR template

cc @Nachiket-Roy @cmainas