Updated test cases for ellipsis error message.
Fixed problem with htdp advanced tests introduced with previous ellipsis updates.
Fixed htdp tests for updated undefined variable error message.
svn: r6887
Uses of these dots expand to an expression generating a runtime error
stating "expected a finished expression, but found a placeholder".
Students may write and run unfinished templates (as usual), but will get an
error message that is more specific and hopefully more useful than unbound variable.
Added tests for this functionality.
svn: r6743
