Handle fcall/nfcall errors correctly. Fixes #33

Hello @x-ma, 

Please review the following commits I made in branch 'nick-fcall'.

3c1527fc4bac2696a31cf70f56782ee04b9b639f (2014-03-20 12:54:09 -0400)
Handle fcall/nfcall errors correctly. Fixes #33

R=@x-ma