Add counters for the number of unhandled errors

Hello @nicks, @majelbstoat, @dpup, 

Please review the following commits I made in branch 'nathan-PromiseStats'.

f23c67992e4a25369391aecda204ad53f09b3024 (2014-05-28 16:27:19 -0700)
Add counters for the number of unhandled errors

R=@nicks
R=@majelbstoat
R=@dpup