Better validation error messages

[barjin]

After having vibecoded an Actor, the apify-cli push failed with a rather cryptic error:

This is because the actor.json file contained the following object:

 "environmentVariables": {
    "KEY": { "secret": true } // this can only be a string
  }

Figuring this out took me some time, as the error message did not mention a filename / code location of the offending String.

Seeing something like Error in actor.json: environmentVariables.KEY - value must be of type string instead would have definitely helped.

[patrikbraborec]

Closing as superseded by #1063 which covers this as part of a broader error messages improvement effort.