how to modify the points 'You are not allowed to perform this operation'

Hi  everyone,

    I recieve a  request from my customer .The scenarios like this:when user get serval  task notifications , then he get a view  of the tasks  by the link

<http://hostName:port/webdynpro/dispatcher/sap.com/tc~bpem~wdui~taskinstance/ATaskExecution?taskId=6fdcaa15dd6d11e2a576000003e0823a>

and some can deal with immediatly ,but others didn't complete .after serval days , user open the task by the link in his email box. then

it come up with a tip : 'a error occurred when  trying  to retrieve  a task , you  are not allowed to perform the operation'.

   I know  the  reason is that  the task haved compeletd  .My customer want to modify the tip, you haved completed the task. I think this is sap built-in 

and we can't modify .Do you have  any good ideas or some  sugguestion . Thanks you very much.