cmd/coordinator: add blank line before SlowBot message

Avoids a problem seen on https://golang.org/cl/269799:

Consult https://build.golang.org/ to see whether they are new failures. Keep in mind that TryBots currently test *exactly* your git commit, without rebasing. If your commit's git parent is old, the failure might've already been fixed.SlowBot builds that ran:

linux-ppc64-buildlet
linux-ppc64le-buildlet

Note that "Slowbot builds" is hidden at the end of the failure message.

Also slightly clean up the code: numFail is the length of a slice, and
can't be negative.

Change-Id: Id9c160dbb7031d75232df54b8662c94b2ce464b6
Reviewed-on: https://go-review.googlesource.com/c/build/+/269919
Trust: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>

cmd/coordinator/coordinator.go

@@ -1519,7 +1519,7 @@ func (ts *trySet) noteBuildComplete(bs *buildStatus) {
 		return
 	}
 
-	const failureFooter = "Consult https://build.golang.org/ to see whether they are new failures. Keep in mind that TryBots currently test *exactly* your git commit, without rebasing. If your commit's git parent is old, the failure might've already been fixed."
+	const failureFooter = "Consult https://build.golang.org/ to see whether they are new failures. Keep in mind that TryBots currently test *exactly* your git commit, without rebasing. If your commit's git parent is old, the failure might've already been fixed.\n"
 
 	s1 := sha1.New()
 	io.WriteString(s1, buildLog)
@@ -1569,14 +1569,15 @@ func (ts *trySet) noteBuildComplete(bs *buildStatus) {
 		if numFail == 0 {
 			score = 1
 			fmt.Fprintf(&buf, "%s are happy.\n", name)
-		} else if numFail > 0 {
+		} else {
 			score = -1
 			ts.mu.Lock()
 			errMsg := ts.errMsg.String()
 			ts.mu.Unlock()
 			fmt.Fprintf(&buf, "%d of %d %s failed:\n%s\n"+failureFooter,
 				numFail, len(ts.builds), name, errMsg)
 		}
+		fmt.Fprintln(&buf)
 		if len(ts.slowBots) > 0 {
 			fmt.Fprintf(&buf, "SlowBot builds that ran:\n")
 			for _, c := range ts.slowBots {