api_dev_fetch_api_key: Improve invalid email message.

Show a user friendly message to the user if email is invalid.
Currently we show a generic message:
"Your username or password is incorrect."

zerver/tests/test_auth_backends.py

@@ -1112,6 +1112,13 @@ class DevFetchAPIKeyTest(ZulipTestCase):
         self.assertEqual(data["email"], self.email)
         self.assertEqual(data['api_key'], self.user_profile.api_key)
 
+    def test_invalid_email(self):
+        # type: () -> None
+        email = 'hamlet'
+        result = self.client_post("/api/v1/dev_fetch_api_key",
+                                  dict(username=email))
+        self.assert_json_error_contains(result, "Enter a valid email address.", 400)
+
     def test_inactive_user(self):
         # type: () -> None
         do_deactivate_user(self.user_profile)

zerver/views/auth.py

@@ -407,6 +407,13 @@ def api_dev_fetch_api_key(request, username=REQ()):
     """
     if not dev_auth_enabled() or settings.PRODUCTION:
         return json_error(_("Dev environment not enabled."))
+
+    # Django invokes authenticate methods by matching arguments, and this
+    # authentication flow will not invoke LDAP authentication because of
+    # this condition of Django so no need to check if LDAP backend is
+    # enabled.
+    validate_login_email(username)
+
     return_data = {} # type: Dict[str, bool]
     user_profile = authenticate(username=username,
                                 realm_subdomain=get_subdomain(request),