Don't attempt to format a non-format string

It can lead to strange output when the string containts a %

log/log.go

@@ -58,3 +58,13 @@ func Errorf(format string, args ...interface{}) {
 	out.WriteString(fmt.Sprintf(format, args...))
 	out.WriteString("\n")
 }
+
+// Error prints the message to stderr, with a red ERROR prefix.
+func Error(msg string) {
+	if level < ErrorLevel {
+		return
+	}
+	out.WriteString(color.RedString("ERROR "))
+	out.WriteString(msg)
+	out.WriteString("\n")
+}

main.go

@@ -29,7 +29,7 @@ func main() {
 		// the same status code
 		os.Exit(ExitCodeWithDefault(err))
 	default:
-		log.Errorf(err.Error())
+		log.Error(err.Error())
 		os.Exit(3)
 	}
 }

testjson/execution.go

@@ -478,7 +478,7 @@ func isGoModuleOutput(scannerText string) bool {
 func parseEvent(raw []byte) (TestEvent, error) {
 	// TODO: this seems to be a bug in the `go test -json` output
 	if bytes.HasPrefix(raw, []byte("FAIL")) {
-		log.Warnf(string(raw))
+		log.Warnf("invalid TestEvent: %v", string(raw))
 		return TestEvent{}, errBadEvent
 	}