Fix rapid7#4307 - Check action for nil

Auxiiary modules already do this, but looks like we forgot to do the
same for post modules.

I also changed the error to allow "reason" in order to be more
informative about what the user should do.

Fix rapid7#4307

Author: wchen-r7

lib/msf/base/simple/auxiliary.rb

@@ -55,7 +55,7 @@ def self.run_simple(omod, opts = {}, &block)
 
     # Verify the ACTION
     if (mod.actions.length > 0 and not mod.action)
-      raise MissingActionError, "You must specify a valid Action", caller
+      raise MissingActionError, "Please use: #{mod.actions.collect {|e| e.name} * ", "}"
     end
 
     # Verify the options

lib/msf/core/exceptions.rb

@@ -210,9 +210,15 @@ def to_s
 ###
 class MissingActionError < ArgumentError
   include AuxiliaryError
+  attr_accessor :reason
+
+  def initialize(reason='')
+    self.reason = reason
+    super
+  end
 
   def to_s
-    "A valid action has not been selected."
+    "Invalid action: #{reason}"
   end
 end
 

lib/msf/core/post.rb

@@ -19,7 +19,13 @@ class Msf::Post < Msf::Module
 
   include Msf::PostMixin
 
-  def setup; end
+  def setup
+    m = replicant
+    action = datastore['ACTION']
+    if m.actions.length > 0 && !m.action
+      raise Msf::MissingActionError, "Please use: #{m.actions.collect {|e| e.name} * ", "}"
+    end
+  end
 
   def type
     Msf::MODULE_POST