firebase-ios-sdk

Log stream errors (#3828)

This is in response to #3817. This is also more consistent with other platforms (e.g., Android [does](https://github.com/firebase/firebase-android-sdk/blob/master/firebase-firestore/src/main/java/com/google/firebase/firestore/remote/AbstractStream.java#L147) log the error).

The resulting log messages look like this:
```
2019-09-11 14:44:03.939778-0400 Firestore_Example_iOS[30490:61949945] 6.8.1 - [Firebase/Firestore][I-FST000001] WriteStream (7f92506abd20) Stream error: 'Not found: No document to update: <document id here, omitted>'
2019-09-11 14:45:01.067452-0400 Firestore_Example_iOS[30490:61949544] 6.8.1 - [Firebase/Firestore][I-FST000001] WriteStream (7f9212de8040) Stream error: 'Permission denied: Missing or insufficient permissions.'
```