Had the same issue. In my case there was no phperrors.log (according to Brandon, it will only exist if there have been any fatal PHP errors)
The error was instead logged to storage/logs/web.log, but even easier, you can view the error in the CP. Craft (as of version 3.0.12) does not show it to the user (hopefully that will change, see https://github.com/craftcms/cms/issues/3012), but currently you can show the errors (as advised by Brandon) by opening your browser's dev tools network tab before running the asset index utility, and keep track of the Ajax requests that get logged when the utility is running (XHR tab).
In my case the error was:
Malformed UTF-8 characters, possibly incorrectly encoded.
and I fixed it by running this on the files:
convmv -r -f windows-1252 -t UTF-8 .