Validate YAML before proceeding

What does this Merge Request do?

This MR will validate YAML before performing any further actions. It will catch invalid YAML as well as invalid settings, e.g.:

Invalid YAML (completely invalid)

gdk.yml

---
broken

$ gdk doctor
(❌) Error: Your gdk.yml is invalid.

undefined method `fetch' for "broken":String

Invalid YAML (partially invalid)

gdk.yml

---
gdk:
  experimental:
    quiet: true

broken

$ gdk doctor
(❌) Error: Your gdk.yml is invalid.

(/Users/ash/src/gdk/scratch1/gdk.yml): could not find expected ':' while scanning a simple key at line 6 column 1

Invalid setting

gdk.yml

---
gdk:
  experimental:
    quiet: tre

$ gdk doctor
(❌) Error: Your gdk.yml is invalid.

Value 'tre' for gdk.experimental.quiet is not a valid bool

Merge Request checklist

• Tests added for new functionality. If not, please raise Issue to follow-up.
• This change is backward compatible. If not, please include steps to communicate to our users.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.

Closes #890 (closed)