Fix Ruby warning about uninitialized instance variable

Instead of trying to get the instance variable and throwing it away to
see if it's there, just check to see if it's there. This fixes the Ruby
warning:

> warning: instance variable @rescue_bindings not initialized

lib/pry-rescue/core_ext.rb

@@ -33,7 +33,7 @@ def rescue(&block)
   #   end
   #
   def rescued(e=$!)
-    if e.instance_variable_get(:@rescue_bindings)
+    if e.instance_variable_defined?(:@rescue_bindings)
       PryRescue.enter_exception_context(e)
     else
       stack = ''
@@ -73,7 +73,7 @@ def rescued(e=$!)
   def enable_rescuing!(block=nil)
     Interception.listen(block) do |exception, binding|
       bindings = binding.respond_to?(:callers) ? binding.callers : [binding]
-      unless exception.instance_variable_get(:@rescue_bindings)
+      unless exception.instance_variable_defined?(:@rescue_bindings)
         exception.instance_variable_set(:@rescue_bindings, bindings)
         exception.instance_variable_set(:@rescue_cause, $!)
       end