pyats validate testbed: Raise all errors at the same time

If there are several errors, the only first error is displayed during the validation. Please check out the sample below:

(pyats) ~/tmp pyats validate testbed  tb.yaml 
Loading testbed file: tb.yaml
--------------------------------------------------------------------------------

devices.vm: Missing keys: ['connections']

(pyats) ~/tmp cat tb.yaml 
devices:
    vm:
      os: 'linux'
      dd:
        f: a
      type: 'linux'
      ddd: ddd
(pyats) ~/tmp 

It will be great if we see all possible errors at the same time.

not exactly easy to fix. there's a lot of recursion on these validation logic.