Catch exceptions only in i-interpolator.

Normal show will propagate the excpetions. Previously, exceptions were filtered
in both cases, which was redundant. Also, it's good to have a way to show things that
does not mask exceptions, if only to debug problems in show itself.

Author: odersky

src/dotty/tools/dotc/printing/Formatting.scala

@@ -28,7 +28,7 @@ object Formatting {
       case arg: Showable =>
         try arg.show(ctx.addMode(Mode.FutureDefsOK))
         catch {
-          case NonFatal(ex) => s"(missing due to $ex)"
+          case NonFatal(ex) => s"[cannot display due to $ex, raw string = $toString]"
         }
       case _ => arg.toString
     }

src/dotty/tools/dotc/printing/Showable.scala

@@ -21,11 +21,7 @@ trait Showable extends Any {
   def fallbackToText(printer: Printer): Text = toString
 
   /** The string representation of this showable element. */
-  def show(implicit ctx: Context): String =
-    try toText(ctx.printer).show
-    catch {
-      case NonFatal(ex) => s"[cannot display due to $ex, raw string = $toString]"
-    }
+  def show(implicit ctx: Context): String = toText(ctx.printer).show
 
   /** The summarized string representation of this showable element.
    *  Recursion depth is limited to some smallish value. Default is