Validation with hook_form_alter()
Adding a validation handler in hook_form_alter() is the Drupal 7 method of adding validation for a field. It will still work in Drupal 8, however the preferred method of adding validation to Field API Field in Drupal 8 is to add a Constraint to the field. Constraints will be applied when the Node is created, no matter the means of creation. Adding a validation handler to the form only validates the field when the node is created using the Form API - IE through a browser. If a node is created by any other means - IE through an API or programatically, the validation is not applied. Constraints are applied no matter the method of creating the node.

Documentation on how to add a constraint can be seen here: https://www.drupal.org/docs/8/api/entity-validation-api/providing-a-custom-validation-constraint

See Method 2 - adding constraints to fields added to an entity you haven't defined, or fields added through the UI