pass err.stack so bunyan actually prints more than "uncaughtException"

I was set up with the wrong value of patchLevel in my config so the server was crashing at startup. Unfortunately, all the error log would say was '"level":60,"op":"uncaughtException","err": {},"msg":"" ...'
which is not the most useful. This change will result in the full stack and root cause ("Error: dbIncorrectPatchLevel") being shown.

`(err && err.stack) || err` is safer