Build Tools Fails Cryptically When User has insufficient Sandboxes

[aweingarten]

Observed Behavior
I tried to set up a new site using build tools:
terminus build:project:create --git=github d8 my-cool-demo

When I had 0 available sandboxes available it fails with a cryptic error:

 [my-cool-demo] Creating a new Pantheon site my-cool-demo
 [CompletionWrapper]  The operation failed to complete.
 [CompletionWrapper]  Exit code 1

Expected Behavior
At the start of the script I should get an error that I lack sufficient sandboxes and then point me to a doc explaining how to see my sandboxes, and how to delete some or get more.