Propagate a better address error

[slackpad]

The unit test cleanup in memberlist ended up burying the real error message if an address isn't available. For example, with some other listener on an address/port normally you'd get an "address already in use" type error, but instead you get this:

==> Error starting agent: Failed to start Consul client: Failed to start lan serf: Failed to create memberlist: Could not set up network transport: ran out of tries to obtain an address

We need to propagate the underlying error.