check-code: automatically preppend "warning: " to all warning messages Some warnings had "warning: " at the beginning of their message. Now this is done consistent for all messages. Especially in test-check-code-hg.t it is an advantage to see warnings at once because only exceptions to them are tolerated. It is (almost) as obvious as before a6180647ea. The prefix will not remain when a warning is changed to a failure. A change like a91387a37f will not be necessary anymore.