fix: convert all RPC error types to exceptions

[plamut]

Fixes #163.

This PR fixes a problem when the underlying gRPC channel is wrapped into something and terminates with a reason that is not an instance of grpc.RpcError. The streaming pull manager now converts all reason types into an Exception instance, fixing a TypeError when a StreamingPullFuture then tries to raise it.

Still waiting for the reporter's confirmation that this is indeed what happens, but I found no other place in the code that could call future.set_exception(...) with a non-Exception argument.

(probably also makes sense to merge #158 first so that we don't have to make yet another release before dropping Python 2)

PR checklist

• Make sure to open an issue as a bug/issue before writing your code! That way we can discuss the change, evaluate designs, and agree on the general idea
• Ensure the tests and linter pass
• Code coverage does not decrease (if any source code was changed)
• Appropriate docs were updated (if necessary)

[pradn]

Looks good, thanks!

[cguardia]

I think this looks good. Thanks!