[chore] Change order of error checking after PostInbox

[tsmethurst]

Check for malformed errors embedded inside error first, then check for gtserror.WithCode.

Description

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This pull request reorders error checking during PostInbox to check for any embedded malformed or actor missing errors first, then fall back to any gtserror.WithCode.

This accounts for cases where a malformed is embedded inside an 500 gtserror. In this case we care more about the embedded 400 as this is more informative for the caller.

closes #3168

Checklist

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• I/we have discussed the proposed changes already, either in an issue on the repository, or in the Matrix chat.
• I/we have not leveraged AI to create the proposed changes.
• I/we have performed a self-review of added code.
• I/we have written code that is legible and maintainable by others.
• I/we have commented the added code, particularly in hard-to-understand areas.
• I/we have made any necessary changes to documentation.
• I/we have added tests that cover new code.
• I/we have run tests and they pass locally with the changes.
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