fix: what bandit and the remaining lint rules found

- nosec for SHA512_password in the login payload (B105): it is a flag
  telling the gateway how Input_Passwd is encoded, not a password; a false
  positive.
- A failed login raises ConnectionException from the underlying error, so
  the cause stays attached.
- tests: pytest.raises instead of try / assert False / except, which
  python -O would have turned into tests that always pass.

napalm_zyxel/zyxel_driver.py

@@ -155,7 +155,8 @@ class ZyxelVMGDriver(ResidentialGatewayDriver):
             "Input_Account": self.username,
             "Input_Passwd": base64.b64encode(self.password.encode("ascii")).decode("ascii"),
             "RememberPassword": 0,
-            "SHA512_password": False,
+            # A flag telling the gateway how Input_Passwd is encoded, not a password.
+            "SHA512_password": False,  # nosec B105
         }
 
         enc_payload = crypto.encrypt_json(login_data)
@@ -169,7 +170,7 @@ class ZyxelVMGDriver(ResidentialGatewayDriver):
             resp.raise_for_status()
             result = crypto.decrypt_response(resp.json())
         except (requests.RequestException, json.JSONDecodeError, TypeError, KeyError) as e:
-            raise ConnectionException(f"Login failed at {self._base_url}: {e}")
+            raise ConnectionException(f"Login failed at {self._base_url}: {e}") from e
 
         if result.get("result") != "ZCFG_SUCCESS":
             raise ConnectionException(