now it's working

agent.rb

module MCollective
    module Agent
        class Mysql<RPC::Agent
            metadata ....

            action "createdb" do
                status = 'Unknown'
                begin
                    >> code that fails for unknown reason <<
                rescue StandardError => e
                    status = "Error: #{e}"
                end
                reply['status'] = status
            end
        end
    end
end