[clang] [clang][doc] Improve error handling for `LibTooling` example code avoiding core dump (PR #98129)

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https://github.com/Sirraide approved this pull request.

`toString(...)` does indeed seem to be the idiomatic way of doing it, but I do wonder if this is intentional: is there ever a reason to log an error but not mark it as handled? I’ll look into this a bit more, but other than that this change seems fine.

https://github.com/llvm/llvm-project/pull/98129