@Pnorman and I have spent a substantial amount of time attempting to debug a fatal error with the following log output:

{"name":"tilerator","hostname":"maps-test2004","pid":14,"level":60,"levelPath":"fatal/service-runner/unhandled","msg":"Cannot read property 'length' of undefined","time":"2018-05-31T19:42:45.546Z","v":0}

This wasn't much to go on, as there are many possible places in any given project (including node modules) where one might try to read the length of something that turns out not to be defined. We worked around it with a monkey-patch to log err.stack (see T194106#4247754). To avoid having to resort to measures like this, logger.js should be updated to give a full stack trace.