Fail fast on invalid playbook

[adatzer]

At the moment, if a playbook has no targets or no steps provided, Run is still called from main.
Since Run returns TargetStatus, missing targets leads to various for loops being skipped, which makes it harder to reason about the return value, and ends up in No queries to run error.
Similarly for steps, which, if missing, []Step is nil, which goes unnoticed till runSteps (after the target client has been created).
Until we get to #149, we could just fail fast instead, with a more relevant error message.