More informative message when dvc.yaml is malformed

[nsorros]

I know we are supposed to let dvc run construct the dvc.yaml but I tend to write them manually as they are quite easy to construct. Often though I am receiving slightly difficult to decipher errors about the format. An example is below

stages:
  train:
    cmd:
      - spacy train config.cfg
    deps:
      - config.cfg
    outs:
      models/spacy-model

this throws ERROR: 'dvc.yaml' format error: extra keys not allowed @ data['stages']['train']['cmd']

it is quite difficult to decipher that this is missing a - in outs. not sure why it complains about cmd either. would it be possible to give back feedback closer to the error?

[dberenbaum]

Related: #5371