tools/syz-aflow: only print errors once at the end

Currently, when an error occurs, it is printed as part of many Spans, in
my experience 3 times:

2026/01/20 19:38:59 finished action crash-reproducer (1/4) in 1.570202281s
results:
        CrashReport:
error:
reproducer did not crash
2026/01/20 19:38:59 finished flow patching (0/0) in 1.571440624s
results:
error:
reproducer did not crash
reproducer did not crash
exit status 1

After this change it is only printed once:

2026/01/20 19:36:42 starting action crash-reproducer (1/4)...
reproducer did not crash
exit status 1

Author: FlorentRevest

tools/syz-aflow/aflow.go

@@ -98,6 +98,11 @@ func run(ctx context.Context, model, flowName, inputFile, workdir string, cacheS
 }
 
 func onEvent(span *trajectory.Span) error {
+	if span.Error != "" {
+		// We do not want to print error twice (once here and once in main).
+		// So we ignore those events.
+		return nil
+	}
 	log.Printf("%v", span)
 	return nil
 }