Show usage instead of error message with tag command.

With the invocation `tag 1` it would print invalid todo number while
todo 1 does exist. It should print the usage text in that case.

test/TagCommandTest.py

@@ -195,6 +195,14 @@ class TagCommandTest(CommandTest.CommandTest):
         self.assertEquals(self.output, "|  4| Fnord\n")
         self.assertEquals(self.errors, "")
 
+    def test_no_tag(self):
+        command = TagCommand(["4"], self.todolist, self.out, self.error)
+        command.execute()
+
+        self.assertFalse(self.todolist.is_dirty())
+        self.assertEquals(self.output, "")
+        self.assertEquals(self.errors, command.usage() + "\n")
+
     def test_help(self):
         command = TagCommand(["help"], self.todolist, self.out, self.error)
         command.execute()

topydo/lib/TagCommand.py

@@ -51,8 +51,10 @@ class TagCommand(Command):
             self.todo = self.todolist.todo(self.argument(0))
             self.tag = self.argument(1)
             self.current_values = self.todo.tag_values(self.tag)
-        except (InvalidCommandArgument, InvalidTodoException):
+        except InvalidTodoException:
             self.error("Invalid todo number.")
+        except InvalidCommandArgument:
+            self.error(self.usage())
 
         try:
             self.value = self.argument(2)