Some (?) ReplicatorChange CouchbaseLiteExceptions contain no message

I haven't exhaustively tested, but taking the below example of an unknown hostname, we get the error code of `5002` which maps to the correct error:

https://github.com/couchbase/couchbase-lite-android/blob/677acb372ce407357eb54eaa41dc81e8b6f40832/shared/src/main/java/com/couchbase/lite/CBLError.java#L67

However this should presumably be included in the `CouchbaseLiteException`, rather than requiring a look up of `5002`:

```
ReplicatorChange{replicator=Replicator[<- Database@3379dd6

{name='********'}

URLEndpoint

{url=ws://*****************:4984/********}

], status=Status{activityLevel=STOPPED, progress=Progress

{completed=0, total=0}

, error=CouchbaseLiteException

{domain='CouchbaseLite', code=5002, msg=null}

}}
```