Do not throw a fatal error when a temp file can't be removed.

It is not that "fatal" since the results won't be affected. Reported by Nesvilab/FragPipe#1512

Author: fcyu

Diff for: lib/met/met.go

@@ -483,7 +483,7 @@ func CleanTemp(dir string) {
 
 	e := os.RemoveAll(dir)
 	if e != nil {
-		msg.Custom(e, "fatal")
+		msg.Custom(e, "warning")
 	}
 }