Revert r6180 as it caused test failures

src/debug.cc

Index: src/debug.cc
===================================================================
--- src/debug.cc	(revision 6182)
+++ src/debug.cc	(working copy)
@@ -622,7 +622,7 @@
 
 // Default call debugger on uncaught exception.
 bool Debug::break_on_exception_ = false;
-bool Debug::break_on_uncaught_exception_ = false;
+bool Debug::break_on_uncaught_exception_ = true;
 
 Handle<Context> Debug::debug_context_ = Handle<Context>();
 Code* Debug::debug_break_return_ = NULL;
@@ -2740,10 +2740,8 @@
   }
 
   if (Socket::Setup()) {
-    if (agent_ == NULL) {
-      agent_ = new DebuggerAgent(name, port);
-      agent_->Start();
-    }
+    agent_ = new DebuggerAgent(name, port);
+    agent_->Start();
     return true;
   }