runner: Deprecate EnsureNoError helper

[wata727]

Closes #196

This PR deprecates runner.EnsureNoError. Instead, use errors.Is() to determine if the error should be ignored:

EDIT: The following example fix has been retracted. Please see #246 for more details.

// before
var val string
err := runner.EvaluateExpr(expr, &val, nil)
err = runner.EnsureNoError(err, func () error {
  // Test values
})
if err != nil {
  return err
}

// after
var val string
err := runner.EvaluateExpr(expr, &val, nil)
if err != nil {
  if errors.Is(err, tflint.ErrUnknownValue) || errors.Is(err, tflint.ErrNullValue) || errors.Is(err, tflint.Sensitive) {
    return nil
  }
  return err
}
// Test values

The above is a simple replacement example, but we recommend that you reconsider whether you should ignore the above errors. For example, instead of ignoring null, you may want to raise an error.

See #196 for a discussion on how it came to be deprecated.

EnsureNoError is not removed at this time, but it will be removed in a future version, so early migration is recommended.