FIX Ensure tab is highlighted for validation error

[GuySartorelli]

Depends on silverstripe/silverstripe-frameworktest#180 for behat to go green.

Issue

• Form field validation/error doesn't highlight the tab where the error occurred silverstripe-cms#2944

[GuySartorelli]

• Keeping validation for projects that might be using that css class as a selector for changed styling or for js logic.
• Need alert-danger for detecting the error in

  silverstripe-admin/client/src/legacy/LeftAndMain.EditForm.js

  Lines 154 to 159 in 16f8e52

  	const $invalidTabPanes = this.find('.tab-pane .alert-danger, .tab-pane .alert.error').closest('.tab-pane');
  	if (!$invalidTabPanes.length) {
  	// If we are at this point it's probably a failed DataObject::validate()
  	// where there was a general (non-field) error added via ValidationResult::addError()
  	return;
  	}

[emteknetnz]

Tested locally, works well

Have merged silverstripe/silverstripe-frameworktest#180 - good to merge when CI has gone green