Write information back somehow to the user of the Addon


Have am workin on a Addon where I do some check up so that the user do not give the wrong infromation in to the code.
It is very easy to use print to show it in the System Console but how do I do so I can see it somewhere else in the Blender UI?
For example the user have given the code wrong filename. I alrade have got code to check it and write out the information to the System Console but if that one is not open. The user can not see theis information.