show a line number when an assertion fails

[hanswesterbeek]

a particular koan, "by-name arguments make lazy arguments" is really giving me trouble as a result :)

[Peeja]

It would also really help to show a meaningful failure, like "Expected 3 but got 4". It's hard to tell what's failing. (I'm new to Scala and ScalaTest, so I don't know if this is a common problem.)