Use short form of keywords in error messages

Some of our error messages reference specific terms, such as:
```
`RootDataEntity` MUST reference `CreateAction` through `schema:mentions`
```

The message should reflect what users should see in the RO-Crate JSON-LD to minimise confusion, so prefixes should be omitted:
```
`RootDataEntity` MUST reference `CreateAction` through `mentions`
```