title: Detect when mat-stepper validation fails
body: <p>We have a linear <code>mat-horizontal-stepper</code> now we want to show a <code>MatSnackBar</code> when the user tries to proceed an forgot a required field.</p>
  
  <p><code>CdkStepper</code> seems to call <code>_anyControlsInvalidOrPending</code> silently and returns if any input is not valid.</p>
  
  <p>Does anybody have an idea how to detect this event?</p>
  
  <p>It must be detected when calling <code>stepper.next()</code>, <code>stepper.previous()</code> and <code>step.select()</code></p>