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

[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:

openapi-generator/modules/openapi-generator/src/main/resources/ruby-client/api_error.mustache

Lines 36 to 37 in d90c9a6

	if @message.nil?
	msg = "Error message: the server returns an error"

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

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@wing328

[wing328]

thanks for the PR.

tests passed via https://app.circleci.com/pipelines/github/OpenAPITools/openapi-generator?branch=ykitamura-mdsol-fix%2Fruby_api_error_message