title: WTforms validate_on_submit ignoring and clearing existing error
body: <p>I have a form that has various fields that are validated in the usual way using validate_on_submit - it works just fine.</p>
  <p>I want to add in  a  'is unique' validation for the form's name field.
  I did not see an easy way to do this is the model so  I did it in post code like this</p>
  <pre><code>if request.method =='POST':
      posted = request.form
      if 'save' in posted:
          # alert has been edited (or is new)
          # check if name is locally unique
          newname = alertform.name.data
          sensordata = site.sensordict[sensorid]
          for alert in sensordata.alert_list:
              if alert.name == newname:
                  err = &quot;Name already in use - please pick another&quot;
                  # need to convert form errors tuple to list, add in new error then convert back to tuple
                  errlist = list(alertform.name.errors)
                  errlist.append(err)
                  alertform.name.errors = tuple(errlist)
                  alertform.form_errors.append(err)
  
  
          if alertform.validate_on_submit():
              # The name.errors and the form_errors are now both empty so validation succeeds!
  </code></pre>
  <p>Stepping through the code I can see I have succeeded in setting the name.errors and form.form_errors when given a dupe name but then running form_validate_on_submit just clears the errors I just set.</p>
  <p>How do I manually add form errors in route code so that validate_on_submit will see them and fail?</p>