Fix bug -- aggregate error(s) from mocha to process exit code of cake

I think this is an indentation bug in testBuilder.coffee --
The err parameter passed to the completion of 'exec' is not being checked.  I think the intention was to check it and return a process exit code of 1 from cake when any tests fail.

Currently, the process exit code of cake is zero even when tests fail.  With the change, the process exit code will be 1 when 1 or more tests fail.

I'm trying to use a Jenkins server now.  When any tests fail, I'd like a process exit code of 1 so that Jenkins can detect the failure.