Add a comment explaining about the error key

Author: wchen-r7

lib/msf/core/rpc/v10/client.rb

@@ -72,6 +72,8 @@ def call(meth, *args)
     if res && [200, 401, 403, 500].include?(res.code)
       resp = MessagePack.unpack(res.body)
 
+      # Boolean true versus truthy check required here;
+      # RPC responses such as { "error" => "Here I am" } and { "error" => "" } must be accommodated.
       if resp && resp.kind_of?(::Hash) && resp['error'] == true
         raise Msf::RPC::ServerException.new(resp['error_code'] || res.code, resp['error_message'] || resp['error_string'], resp['error_class'], resp['error_backtrace'])
       end