Need to add "Unknown exception" to retryable mixin

[inertia186]

BaseError needs to handle Unknown exception in addition to the others.

Note, hivemind is moving away from a whitelist due to maintenance concerns.

AC

• [ ] lib/steem/base_error.rb handles Unknown exception as Steem::UnknownException, similar to the other three mentioned in https://github.com/steemit/hivemind/issues/126#issuecomment-397332016
• [ ] lib/steem/mixins/retriable.rb is updated to include Steem::UnknownException in RETRYABLE_EXCEPTIONS
• [ ] lib/steem/rpc/http_client.rb might also include these or a subset.

[inertia186]

This whole thing is a bit of a facepalm because the upstream (jussi or something) might respond with ambiguous errors, handled by accurately named error classes.

But which one's which?

I just wanted to clarify, the commit references Steem::UnknownExceptionError but the issue references Steem::UnknownException. The actual error is indeed intended to be Steem::UnknownExceptionError.

The true error is Steem::UnknownExceptionError because it whitelists the error message "Unknown Exception" from upstream. Getting the text "Unknown Exception" from the upstream API is a different situation from getting a new error message that steem-ruby has never encountered before (aka: Steem::UnknownError).

The rule is simple:

• Doesn't exist, it was just a typo in the original issue: Steem::UnknownException
• Should probably retry: Steem::UnknownExceptionError
• Should never retry: Steem::UnknownError