ENHANCEMENT Validate against missing block division

[tractorcow]

E.g. in this config

---
Name: myemailconfig
After:
  - '#emailconfig'
SilverStripe\Core\Injector\Injector:
  Swift_Transport:
    class: Swift_SmtpTransport
    properties:
      Host: mail.example.com
      Port: 587
      Encryption: null
    calls:
      Username: [ setUsername, ['example@example.com'] ]
      Password: [ setPassword, ['123456'] ]
      AuthMode: [ setAuthMode, ['login'] ]

The missing --- on the block header groups this entire file inside the header (with no body) and thus acts as a blank config block. However, no error is raised.

We should raise a warning if an unsupported header key is presented, with a whitelist of allowed keys (e.g. name, only, after, before, except).

Noted in silverstripe/silverstripe-framework#7700 (comment)

[chillu]

Just noting that it's silently breaking on a missing start header division as well, which just cost me 30 mins of goose chasing :/ We need more explicit validation in the whole config layer, it's a massive dev time sink right now.

[chillu]

Marking as impact/high since those dev experience failures suck up lots of time (and willpower) across the ecosystem