[py] correctly handle additional data in unexpected alter error for w3c drivers (#5416)

The WebDriver spec defines an optional "data" field for error objects
and annotated unexpected alert open error which includes a "text" field
in the error additional data object.

https://w3c.github.io/webdriver/webdriver-spec.html#dfn-error-data
https://w3c.github.io/webdriver/webdriver-spec.html#dfn-user-prompt-handler

Author: carlosgcampos

py/selenium/webdriver/remote/errorhandler.py

@@ -232,8 +232,13 @@ def check_response(self, response):
                 pass
         if exception_class == ErrorInResponseException:
             raise exception_class(response, message)
-        elif exception_class == UnexpectedAlertPresentException and 'alert' in value:
-            raise exception_class(message, screen, stacktrace, value['alert'].get('text'))
+        elif exception_class == UnexpectedAlertPresentException:
+            alert_text = None
+            if 'data' in value:
+                alert_text = value['data'].get('text')
+            elif 'alert' in value:
+                alert_text = value['alert'].get('text')
+            raise exception_class(message, screen, stacktrace, alert_text)
         raise exception_class(message, screen, stacktrace)
 
     def _value_or_default(self, obj, key, default):