Better handling of incorrect version for validate

[squaremo]

src/validate.js reads a parameter for the Kubernetes version to validate against; since it uses that directly as a directory name, the error that comes back if you get it wrong is a bit mysterious.

• make sure it's a valid version and the directory exists
• consider correcting some common misspellings, e.g., 1.17.0 instead of v1.17.0.