don't return exit code 0 on failure

we need to track total/pass like the other reporters, since that's what
testem uses to infer success/failure:

https://github.com/airportyh/testem/blob/85efed9/lib%2Fci%2Findex.js#L157
Merge pull request #1 from jenseng/exitcode

don't return exit code 0 on failure