Do not show stacktraces on some intentionally-thrown errors (#9056)

* Do not show stacktraces on some intentionally-thrown errors

* Fix pylint error

* Fix JavaScript looking for the wrong key in error response from connection check

* format -> f-string

* Run black because PyCharm missed it on the last save

tests/core_tests.py

@@ -418,6 +418,26 @@ class CoreTests(SupersetTestCase):
         assert response.status_code == 200
         assert response.headers["Content-Type"] == "application/json"
 
+    def test_testconn_failed_conn(self, username="admin"):
+        self.login(username=username)
+
+        data = json.dumps(
+            {"uri": "broken://url", "name": "examples", "impersonate_user": False}
+        )
+        response = self.client.post(
+            "/superset/testconn", data=data, content_type="application/json"
+        )
+        assert response.status_code == 400
+        assert response.headers["Content-Type"] == "application/json"
+        response_body = json.loads(response.data.decode("utf-8"))
+        expected_body = {
+            "error": "Connection failed!\n\nThe error message returned was:\nCan't load plugin: sqlalchemy.dialects:broken"
+        }
+        assert response_body == expected_body, "%s != %s" % (
+            response_body,
+            expected_body,
+        )
+
     def test_custom_password_store(self):
         database = utils.get_example_database()
         conn_pre = sqla.engine.url.make_url(database.sqlalchemy_uri_decrypted)