Improve error message in `timew modify`

Follow-up for the side-issue in #321

Assuming that there is a stopped interval in @1:

```sh
❯ timew modify end @1 20200629T10:40
No updated time specified. See 'timew help modify'.
```

This should report that the time format is wrong, not that there is nothing.

Start by reproducing the documented `timew modify end @1 20200629T10:40` command and trace the `timew modify` handling of the supplied time. Done means this invalid time format produces an appropriate format error instead of “No updated time specified”; consult any existing validation tests if available.

Written by the indexing model from the issue text.