more informative error message if test fails due to warnings
more informative error message if test fails due to warnings

robzhu/r-travis,robzhu/r-travis,eddelbuettel/r-travis,qinwf/r-travis,eddelbuettel/r-travis,hadley/r-travis,craigcitro/r-travis,hadley/r-travis,craigcitro/r-travis,qinwf/r-travis