rose_arch: friendlier error messages

This alters the error messages for having:

An invalid `command-format` value:
```
command-format=eat the raspberry %(source)s donut %(target)s
```
 which currently fails with:
```
KeyError: 'source'
```
  should now fail with a more descriptive message:
```
ConfigValueError: command-format=eat the raspberry %(source)s donut %(target)s: configuration value error: 'source'
```

A missing compulsory item such as `source=...`, which currently reports:
```
ConfigValueError: arch=None: configuration value error: 'source'
```
 should now report:
```
CompulsoryConfigValueError: source=None: missing configuration error: 'source'
```


@matthewrmshin, please review.