Checks if the target files can be opened.
        Any files that cannot be opened are removed from the self.target_files list.

def _open_target_files(self):
        '''
        Checks if the target files can be opened.
        Any files that cannot be opened are removed from the self.target_files list.
        '''
        # Validate the target files listed in target_files
        for tfile in self.files:
            # Ignore directories.
            if not self.subclass == io.FileIO or not os.path.isdir(tfile):
                # Make sure we can open the target files
                try:
                    fp = self.open_file(tfile)
                    fp.close()
                    self.target_files.append(tfile)
                except KeyboardInterrupt as e:
                    raise e
                except Exception as e:
                    self.error(description="Cannot open file %s (CWD: %s) : %s" % (tfile, os.getcwd(), str(e)))