Expose setting for sendmail failure alerts

tailbone/templates/settings/email/configure.mako

@@ -15,6 +15,15 @@
       </b-checkbox>
     </b-field>
 
+    <b-field>
+      <b-checkbox name="rattail.mail.send_email_on_failure"
+                  v-model="simpleSettings['rattail.mail.send_email_on_failure']"
+                  native-value="true"
+                  @input="settingsNeedSaved = true">
+        When sending an email fails, send another to report the failure
+      </b-checkbox>
+    </b-field>
+
   </div>
 
   % if request.has_perm('errors.bogus'):

tailbone/views/email.py

@@ -34,6 +34,7 @@ import six
 from rattail import mail
 from rattail.db import model
 from rattail.config import parse_list
+from rattail.util import simple_error
 
 import colander
 from deform import widget as dfwidget
@@ -283,6 +284,9 @@ class EmailSettingView(MasterView):
             {'section': 'rattail.mail',
              'option': 'record_attempts',
              'type': bool},
+            {'section': 'rattail.mail',
+             'option': 'send_email_on_failure',
+             'type': bool},
         ]
 
     def toggle_hidden(self):
@@ -414,11 +418,14 @@ class EmailPreview(View):
                 email = self.email_handler.get_email(key)
                 data = email.obtain_sample_data(self.request)
 
+                try:
                     self.email_handler.send_message(email, data,
                                                     subject_prefix="[PREVIEW] ",
                                                     to=[recipient],
                                                     cc=None, bcc=None)
-
+                except Exception as error:
+                    self.request.session.flash(simple_error(error), 'error')
+                else:
                     self.request.session.flash(
                         "Preview for '{}' was emailed to {}".format(
                             key, recipient))