Avoid failing early when we cannot find symbols on macho

[tkonolige]

I've been hitting this issue on macOS where backtrace errors out early even though it can still find symbols for some libraries. This happens when I am loading my library dynamically from python, so I assume its some python libraries that are causing issues.

I'm not quite sure if the logic here is correct. It works in my use cases, but I think I need someone who knows the codebase to verify it is correct.

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[tkonolige]

@googlebot I signed it!

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[tkonolige]

@ianlancetaylor Could you take a look at this. Its a pretty simple fix.