a couple small plovr fixes

Hello me, 

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

93a1147a4796a37140aa21e8223c5b85867b17d5 (2013-05-15 14:19:17 -0400)
Use closure-compiler's error-handling, instead of re-inventing the wheel

38ba9df1d0de4307350c72b1494c70aa15b22a69 (2013-05-15 14:19:17 -0400)
There's no good reason to access compiler Result directly

d8360de314906f7fd64692d640bb7f74cb333c9e (2013-05-15 14:19:17 -0400)
Fix up plovr tests

62df746f334c422ea12c2db8bd707ac9fef4e1a1 (2013-05-15 14:19:17 -0400)
Use rhino properly, so that this works on openjdk

R=me