fix: handle dead symlinks in walk

[markjm]

Fixes #549

The issue is due to an unhandled error bubbling in the case where a symlink exists but the target has been deleted.

Two other things to note here, which may just show my naiveté w Go (this is my first project working with it):

1. I am unsure if there is a standard/best practice for error handling here. Seems errors can bubble/disappear really easily right now
2. A fix like this would benefit from a regression test, but I am not too sure how the testing in this project is handled. I think even a small doc on that would be valuable. I am happy to help put that together or contribute.

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