Suppress TimeMachine Connection Errors
While I haven't been able to suppress the message I have found a solution to this and other location based problems.

I use an app called Control Plane, which is open source donationware, that senses where I am and changes any number of settings based on the location.

There is also a more fully featured app called Sidekick available for $29 US. Federico Vitti has a good overview of it here.