The alert send functions (Slack/Teams/GChat/raw) return `[]error`, but
`SendWebhookAlert` discarded them, so a failing webhook — e.g. a Teams
alert returning a non-2xx status — produced no output at all, even at
trace level (#949).
Capture the returned errors and log each with `logrus.Errorf`, as
suggested by the maintainer on the issue. Adds a regression test that
drives a failing (500) webhook and asserts the error is logged.
Closes#949
Assisted-by: Claude Code (Anthropic, Opus 4.x)
