* src/filetree.{cc,h} (FileTree::find_add_file): New method.
Move the actual file-adding parts from find_check_file() here.
(FileTree::find_recursively): Move the file testing previously
done in find_check_file() into this method in order to make the
recursion more obvious.  Also handle Glib::Dir exceptions right
here, so that the caller doesn't have to handle them.
(FileTree::find_files): Remove the try/catch block enclosing the
call to find_recursively().  We no longer need to do it twice.
(FileTree::find_check_file): Remove.