fix: exit the goose and show the error message when provider environment variable is not set

[lifeizhou-ap]

Why

Currently it throws a runtime error and print the stack trace when

• when the environment variable is not set for LLM apis,
• when unknown provider or moderator is trying to be loaded

This make it hard for users to understand what is going.

What

Exchange

• Raised a MissingProviderEnvVariableError with attributes of provider, env_variable. When providers fetch the environment variable for api key, it will raise this error if the environment variable is not available
• Raised a LoadExchangeAttributeError for unknown provider or moderator

With the above custom erross we can construct user friendly messages in goose

goose

• Raised a LoadExchangeAttributeError for unknown provider or toolkit
• Catch the custom errors Show the error message without stacktrace and exit the program.

screenshots

Please give feedback including the content and presentation of these error message. Thank you!