flatten of `try ...; catch; false; end` does not work

[Krastanov]

Even with #195 the following still does not work:

quote; try; error(); catch; false; finally; println(123); end; end |> striplines |> flatten |> eval

The flattening of the catch leads to a false becoming the third argument of the try expression, which can not be distinguished from a try ...; finally block without a catch statement. Previously this was a silent bug. With #195 now it explicitly raises an error with a suggestion for how to rework the statement.

A complete fix needs a different tree traversal strategy and a rewrite of flatten. This issue is submitted here in order to keep record of the implemented changes and steps necessary for a complete fix.