Add file validation

Author: rastating

payloads/custom.rb

@@ -22,8 +22,21 @@ def encoded
       raw
     end
 
+    def payload_path
+      File.expand_path(datastore['payload_path'])
+    end
+
+    def prepare(mod)
+      unless File.exist?(payload_path)
+        mod.emit_error "#{payload_path} does not exist"
+        return false
+      end
+
+      true
+    end
+
     def raw
-      File.open(File.expand_path(datastore['payload_path']), 'rb', &:read)
+      File.open(payload_path, 'rb', &:read)
     end
   end
 end