Both the Historical and Event drivers have the statusmessage variable that can be used to indicate any issues when making a request.

Simply set the variable to a value that describes the problem (ie. 'Access Denied') to give feedback to the user when a particular action could not be performed.

For example…

try:
   #Open and read the file contents
   f = open(self.filename,'r')
   content = f.read()
   f.close()
except:
   #Tell the user the file could not be opened.
   self.statusmessage = "Could Not Open File"