Add a validation check to Settings (#8019)

This is for consistency with other platforms (e.g. [Android](https://github.com/firebase/firebase-android-sdk/blob/3f0779ede98e4aa6986844a767c7f0eebc9d19b5/firebase-firestore/src/main/java/com/google/firebase/firestore/FirebaseFirestoreSettings.java#L141-L144)).

Author: var-const

Firestore/core/src/api/firestore.cc

@@ -111,7 +111,7 @@ void Firestore::set_settings(const Settings& settings) {
   }
   if (!settings.ssl_enabled() && settings.host() == Settings::DefaultHost) {
     util::ThrowIllegalState(
-        "You can't set the 'sslEnabled' setting unless you also set a "
+        "You can't set the 'sslEnabled' setting to false unless you also set a "
         "non-default 'host'.");
   }