Xcode bot error: Early unexpected exit, operation never finished bootstrapping
I was facing the same error (although I was using fastlane to run my UI Tests).
The issue was due to the simulator being stuck on a system alert which the UI Tests did not handle. After that all my tests failed and subsequent runs continued to fail as the simulator was not responding.
I logged into the machine and click OK on the disrupting alert and everything was back to normal.
In my case the alert came up since the application was trying to register of APNS and the
"App Name" would like to enable Notifications alert was displayed. Skipping the APNS registration for UI Tests prevented the issue from popping again.