chore: Improve error message for missing API key.

PiperOrigin-RevId: 875248443

Author: Annhiluc

google/genai/_api_client.py

@@ -704,9 +704,9 @@ def __init__(
     else:  # Implicit initialization or missing arguments.
       if not self.api_key:
         raise ValueError(
-            'Missing key inputs argument! To use the Google AI API,'
-            ' provide (`api_key`) arguments. To use the Google Cloud API,'
-            ' provide (`vertexai`, `project` & `location`) arguments.'
+            'No API key was provided. Please pass a valid API key. Learn how to'
+            ' create an API key at'
+            ' https://ai.google.dev/gemini-api/docs/api-key.'
         )
       self._http_options.base_url = 'https://generativelanguage.googleapis.com/'
       self._http_options.api_version = 'v1beta'