Unexpected unknown-error is returned

[hankovich]

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

[google-oss-bot]

I found a few problems with this issue:

• I couldn't figure out how to label this issue, so I've labeled it for a human to triage. Hang tight.
• This issue does not seem to follow the issue template. Make sure you provide all the required information.