Improve error messages in "image not found" case

[reliveyy]

# Case 1. in setup.sh
Image not found in https://registry-1.docker.io/: exchangeunion/utils:latest__stream, exchangeunion/utils:latest

# Case 2. in python launcher
Image(s) not found: exchangeunion/xud:1.2.1.

Sometimes we cannot get a proper response when checking a registry image but it could be the network issue. So the "image not found" error should be restricted to API 404 HTTP error response only.