SI-7110 Warn about naked try without catch/finally

Before, this was allowed:

    scala> try ( 1 / 0 )
    java.lang.ArithmeticException: / by zero

But since the advent of util.Try, the subtle difference to the
following seems dangerous:

    scala> import util.Try
    import util.Try

    scala> Try ( 1 / 0 )
    res4: scala.util.Try[Int] = Failure(java.lang.ArithmeticException: / by zero)

Discussion: https://groups.google.com/d/topic/scala-language/fy2vXD_3fF8/discussion

There was some concern that this curtails a handy, temporary
way to remove the exception handlers from some code. But after
thinking about this, I contend that:

 a) those people can easily stomach the warning temporarily
    (modulo, of course, those with -Xfatal-warnings.)
 b) putting this warning behind Xlint will disable it for those
    who need it most: beginners.

I also chose not to refer to 'scala.util.Try' in the error message
as I think that has as much potential to confuse as it does to clarify.
Merge pull request #2337 from retronym/ticket/7110-2

SI-7110 Warn about naked try without catch/finally