Issue #249: error messages involving postings should start counting at 1instead of 0 (blais/beancount)

[Martin Michlmayr]

New issue 249: error messages involving postings should start counting at 1instead of 0
https://bitbucket.org/blais/beancount/issues/249/error-messages-involving-postings-should

Martin Michlmayr:

I got:

```
Failed to categorize posting 0
```

I know programmers like to start counting at 0 but wouldn't 1 be more intuitive here?