Slightly better error message when user protects a non-existent commit

See eg https://github.com/rtyley/bfg-repo-cleaner/issues/47

Better than this, anyway:

```
[error] (run-main-5) java.lang.NullPointerException
java.lang.NullPointerException
	at org.eclipse.jgit.lib.ObjectIdOwnerMap.get(ObjectIdOwnerMap.java:131)
	at org.eclipse.jgit.revwalk.RevWalk.parseAny(RevWalk.java:807)
	at com.madgag.git.package$RichObjectId.asRevObject(package.scala:190)
	at com.madgag.git.bfg.cleaner.protection.ProtectedObjectCensus$$anonfun$3.apply(ProtectedObjectCensus.scala:66)
```