Add check to ensure there are no errors, in addition to no failures,
in the test driver to ensure R CMD CHECK fails.  Rcpp 0.11.1 now
doesn't want S4 objects to be R_NilValue which we previously depended
on.

RProtoBuf 0.4.1 has this bug and the unit test even gives a runtime
error, but this driver wasn't reporting the failure.



git-svn-id: svn://svn.r-forge.r-project.org/svnroot/rprotobuf@892 edb9625f-4e0d-4859-8d74-9fd3b1da38cb