flo should confirm that `creates` exists after a task is run

This came up with @stringertheory modified #58 and inadvertently did something like this:

```yaml
---
creates: a_file.txt
command: echo "hello world" > {{creates[0]}} # creates file called a
---
creates: b_file.txt
command: echo "hello world" > {{creates[1]}} # creates file called _
```

flo should detect when a `creates` isn't actually created and throw an error.