[ruby] Fix api_error.mustache to initialize message-only errors properly

Created by: ykitamura-mdsol

When generating message-only errors (e.g. ApiError.new('Connection failed')), the @message instance variable is not set so the message method returns the default message:

https://github.com/OpenAPITools/openapi-generator/blob/d90c9a6f3bdabec92019e7a78837400c575c5735/modules/openapi-generator/src/main/resources/ruby-client/api_error.mustache#L36-L37

With this PR (setting the @message instance variable), the message method returns the expected message:

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@wing328