Where can I find the meaning of NUnit console exit codes?

Error code -100 stands for UNEXPECTED_ERROR

static ConsoleUi()
{
    OK = 0;
    INVALID_ARG = -1;
    FILE_NOT_FOUND = -2;
    FIXTURE_NOT_FOUND = -3;
    TRANSFORM_ERROR = -4;
    UNEXPECTED_ERROR = -100;
}

EDIT:
Additional information from a thread on the NUnit-Discuss google group:

Additionally, positive values give a
count of failed tests in the run.

The -100 return code is a catch-all,
usually indicating an unhandled
exception in your application or test.
It should normally come with a stack
trace.