serve an error when tor is missing
TorBrowser serves user with a popup when tor exit early, for instance due to invalid configuration, however it does not appear to do so when tor can't be spawned at all.
This situation happened recently/is happening for many users due to WindowsDefender deleting their tor.exe. The result they get seems somewhat similar to an ISP blocking tor (including bridges). Only the absence of tor logs give it away, but you have to know what to look for.