report all errors from module validation

[jbardin]

It can be tedious fixing a new module with many errors when Terraform
only outputs the first random error it encounters.

Accumulate all errors from validation, and format them for the user.

Example output of multiple variables missing in modules:

  module root:
    module mod: required variable in_a not set
    module mod: required variable in_b not set
    module mod: required variable in_c not set
  module mod.root:
    module mod2: required variable in2_c not set
    module mod2: required variable in2_a not set
    module mod2: required variable in2_b not set

[ghost]

I'm going to lock this issue because it has been closed for 30 days ⏳. This helps our maintainers find and focus on the active issues.

If you have found a problem that seems similar to this, please open a new issue and complete the issue template so we can capture all the details necessary to investigate further.