fix error

lib/msf/base/simple/exploit.rb

@@ -151,10 +151,10 @@ module Exploit
       if e.kind_of?(Msf::OptionValidateError)
         dlog("Call stack:\n#{e.backtrace.join("\n")}", 'core', LEV_3)
       else
-        mod.print_error("Call stack:")
+        exploit.print_error("Call stack:")
         e.backtrace.each do |line|
           break if line =~ /lib.msf.base.simple.exploit.rb/
-          mod.print_error("  #{line}")
+          exploit.print_error("  #{line}")
         end
         elog("Call stack:\n#{e.backtrace.join("\n")}", 'core', LEV_0)
       end