Improve error messages when a github repo already exists

[aweingarten]

Observed Behaviors
When I try to create a new project

• if the pantheon site does not exist
• the github repo already exists
• terminus build:project:create --git=github d8 my-demo
• I see a cryptic error that doesn't tell me much

Expected behavior
I expect that before the setup script gets very far it checks that the github repo does not exist and if it does then provides meaningful user feedback.

Why this is a valid use case
The setup process is a giant monolith, if any part fails its really hard for a use to restart the process and if they do they hit left over land mines from their previous runs.