added system.onRaise to support a condition system

lib/system/excpt.nim

@@ -52,6 +52,7 @@ proc setFrame(s: PFrame) {.compilerRtl, inl.} =
   framePtr = s
 
 proc pushSafePoint(s: PSafePoint) {.compilerRtl, inl.} = 
+  s.raiseAction = nil
   s.prev = excHandler
   excHandler = s
 
@@ -198,8 +199,9 @@ proc raiseExceptionAux(e: ref E_Base) =
   if globalRaiseHook != nil:
     if not globalRaiseHook(e): return
   if excHandler != nil:
-    pushCurrentException(e)
-    c_longjmp(excHandler.context, 1)
+    if isNil(excHandler.raiseAction) or excHandler.raiseAction(e):
+      pushCurrentException(e)
+      c_longjmp(excHandler.context, 1)
   elif e[] of EOutOfMemory:
     writeToStdErr(e.name)
     quitOrDebug()