Unexpected unknown-error is returned

I use `SubscribeToTopicAsync` method to add tokens to topic and sometimes get errors. Some of them are easy to understand (`registration-token-not-registered`), but sometimes I see undocumented `unknown-error`. I do not how to handle it and my background job makes more and more attempts to add such tokens to topic.

I suggest
1. Make constants like `invalid-argument`, `registration-token-not-registered` public. Or (which is event better) at as a simple proxy, do not add any additional error mapping, do not add any layers and abstractions over the fcm api
2. Do not swallow info about unknown errors for my code to be able to handle them. For now I do not known what to do with such errors. Are they retriable, are they permanent? Who knows