Proper C scopes for if, try and case statements

even thought the setjmp implementation for try statement won't be used in C++,
using properly scoped variables for them is beneficial, because we'll be able to
establish a 1:1 relation between nimrod scopes and C scopes.
Once we have that, we'll be able to keep the original names of local variables
to greatly improve the debugging experience (i.e. watch expressions and
hover tooltips will work).

lib/system.nim

@@ -1593,12 +1593,12 @@ proc echo*[Ty](x: openarray[Ty]) {.magic: "Echo", noSideEffect.}
 template newException*(exceptn: typeDesc, message: string): expr = 
   ## creates an exception object of type ``exceptn`` and sets its ``msg`` field
   ## to `message`. Returns the new exception object. 
-  block: # open a new scope
-    var
-      e: ref exceptn
-    new(e)
-    e.msg = message
-    e
+  # block: # open a new scope
+  var
+    e: ref exceptn
+  new(e)
+  e.msg = message
+  e
 
 when not defined(EcmaScript) and not defined(NimrodVM):
   {.push stack_trace: off.}