Abort execution when terraform fails

[betabandido]

In some case the result of executing terraform is not checked (e.g., see

TerraformDevKit/tasks/devkit.rake

Line 98 in 28b0ecd

system("terraform plan -out=#{PLAN_FILE}")

).

Fix that. Ideally, we could use the latest implementation of Command.run (available in feature/use-fixed-command).