Fix rapid7#4047 - undefined method `rank' due to an invalid encoder name

Fix rapid7#4047 caused by an invalid encoder name. Also added elog() to
avoid shutting everything up in msfvenom

Author: wchen-r7

lib/msf/core/payload_generator.rb

@@ -308,12 +308,14 @@ def get_encoders
       if encoder.present?
         # Allow comma seperated list of encoders so users can choose several
         encoder.split(',').each do |chosen_encoder|
-          encoders << framework.encoders.create(chosen_encoder)
+          e = framework.encoders.create(chosen_encoder)
+          encoders << e if e
         end
         encoders.sort_by { |my_encoder| my_encoder.rank }.reverse
       elsif badchars.present?
         framework.encoders.each_module_ranked('Arch' => [arch], 'Platform' => platform_list) do |name, mod|
-          encoders << framework.encoders.create(name)
+          e = framework.encoders.create(name)
+          encoders << e if e
         end
         encoders.sort_by { |my_encoder| my_encoder.rank }.reverse
       else

msfvenom

@@ -300,6 +300,7 @@ if __FILE__ == $0
     venom_generator =  Msf::PayloadGenerator.new(generator_opts)
     payload = venom_generator.generate_payload
   rescue ::Exception => e
+    elog("#{e.class} : #{e.message}\n#{e.backtrace * "\n"}")
     $stderr.puts e.message
   end