fix: fix the example code of error handling

[yixuan]

Description

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According to the class structure, error classes such as APIStatusError and APITimeoutError are not accessible via zai.* as in the example code in README:

from zai import ZaiClient
import zai

client = ZaiClient(api_key="your-api-key")

try:
    response = client.chat.completions.create(
        model="glm-4",
        messages=[
            {"role": "user", "content": "Hello, Z.ai!"}
        ]
    )
    print(response.choices[0].message.content)
    
except zai.APIStatusError as err:
    print(f"API Status Error: {err}")
except zai.APITimeoutError as err:
    print(f"Request Timeout: {err}")
except Exception as err:
    print(f"Unexpected Error: {err}")

Instead, the correct path is zai.core.APIStatusError and zai.core.APITimeoutError, etc.