optimize error message when validating api_key failed

[usamimeri]

Features
Added a more user-friendly error message for API_KEY validation failure for #1290, #1322 and #1220.
Users can use metagpt --init-config to add a config2.yaml under home/username/.metagpt/config2.yaml.
However, the config file in the home directory will overwrite the one in the MetaGPT repository, which may cause unexpected results. Therefore, the error message has been optimized.
Result

1. When two paths exist or only the config under home path exists and there is no api_key in the home path one:

ValidationError: 1 validation error for Config
llm.api_key
  Value error, Please set your API key in /home/usamimeri/.metagpt/config2.yaml. If you also set your config in /home/usamimeri/MetaGPT/config/config2.yaml, 
the former will overwrite the latter. This may cause unexpected result.
 [type=value_error, input_value='YOUR_API_KEY', input_type=str]
    For further information visit https://errors.pydantic.dev/2.6/v/value_error

2. When only the config under the repository exists:

ValidationError: 1 validation error for Config
llm.api_key
  Value error, Please set your API key in /home/usamimeri/MetaGPT/config/config2.yaml [type=value_error, input_value='YOUR_API_KEY', input_type=str]
    For further information visit https://errors.pydantic.dev/2.6/v/value_error