simplify validation errors

[pomdtr]

Our current validation error message are hard to craft, and still do not provide a good user experience.

Error: Spacefile
  L Micro 'static-app'
    L Micro 'static-app'
      L Micro 'static-app'
        L missing properties: 'serve'

Ideally, we want something like like this

However, it is not trivial to achieve. In the meantime, I think this is a good middleground:

Error: validation failed:
L /micros/static-app: missing properties: 'serve'