send_test_email: Throw an error if email not configured.

This should help minimize confusion when folks try to use this before
properly configuring outgoing email.

Thanks to Bruce Eckel for the report.

zerver/management/commands/send_test_email.py

@@ -1,13 +1,18 @@
 
 from typing import Any
 
+from django.conf import settings
 from django.core.mail import mail_admins, mail_managers, send_mail
+from django.core.management import CommandError
 from django.core.management.commands import sendtestemail
 
 from zerver.lib.send_email import FromAddress
 
 class Command(sendtestemail.Command):
     def handle(self, *args: Any, **kwargs: str) -> None:
+        if settings.WARN_NO_EMAIL:
+            raise CommandError("Outgoing email not yet configured, see\n  "
+                               "https://zulip.readthedocs.io/en/latest/production/email.html")
         subject = "Zulip Test email"
         message = ("Success!  If you receive this message, you've "
                    "successfully configured sending email from your "

zerver/tests/test_management_commands.py

@@ -28,6 +28,11 @@ class TestCheckConfig(ZulipTestCase):
             with self.assertRaisesRegex(CommandError, "Error: You must set asdf in /etc/zulip/settings.py."):
                 check_config()
 
+    @override_settings(WARN_NO_EMAIL=True)
+    def test_check_send_email(self) -> None:
+        with self.assertRaisesRegex(CommandError, "Outgoing email not yet configured, see"):
+            call_command("send_test_email", 'test@example.com')
+
 class TestZulipBaseCommand(ZulipTestCase):
     def setUp(self) -> None:
         self.zulip_realm = get_realm("zulip")