tags: Log stderr message when mktag command failed

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Addresses the issue reported in Issue #6681.

I initially planned to add a stderr message to the original error so it could be handled at a higher level. However, after seeing the comment about backward compatibility, I decided not to modify the original error, as I wasn't sure where that requirement was coming from:

func (e MktagError) Error() string {
	// TODO: Upper-case error message purely for transitory backwards compatibility
	return fmt.Sprintf("Could not update refs/tags/%s. Please refresh and try again.", e.tagName)
}

To avoid any potential issues, I chose to log the error message instead of modifying the original error. This way, we can still capture the error details without breaking compatibility.