[services] Backup - return error when inner client reactor fails

Summary:
Depends on D3804

We need to propagate =errors from the client reactors that are spawned in the server reactors.

Test Plan: The same as in D3535 but this time perform actions from the gRPC client without starting a blob service and see that the client receives proper errors indicating that there's a connection problem.

Reviewers: palys-swm

Reviewed By: palys-swm

Subscribers: ashoat, palys-swm, Adrian, atul, benschac, yayabosh

Differential Revision: https://phabricator.ashoat.com/D3722