fix(dataposter): fail fast on invalid root CA PEM

[maximevtush]

Adds validation of AppendCertsFromPEM return value when loading external signer root CA.
Previously, an empty or malformed PEM file would silently result in an empty CA pool, causing obscure TLS handshake failures later.

[github-actions]

Thank you for your contribution. However, this PR has been automatically closed because it was labeled as trivial. As stated in our CONTRIBUTING.md:

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Here are some examples of trivial PRs that will most-likely be rejected:

• Fixing typos
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