1.6.3 branch - Improved error message in Checkers.scala.
1.6.3 branch - Improved error message in Checkers.scala.

git-svn-id: 8ae76d6256a5acd2cad8ab2ed281e34e70247003@3151 fd1274fd-080f-5963-8033-43ee40fcd1de

yyuu/scalatest,vivosys/scalatest,vivosys/scalatest,yyuu/scalatest