Log message to stderr if LOG_FILE is not writable

Co-authored-by: Helmut K. C. Tessarek <tessarek@evermeet.cx>

src/config.rs

@@ -690,6 +690,12 @@ fn validate_config(cfg: &ConfigItems) -> Result<(), Error> {
         }
     }
 
+    if let Some(log_file) = &cfg.log_file {
+        if std::fs::OpenOptions::new().append(true).create(true).open(log_file).is_err() {
+            err!("Unable to write to log file", log_file);
+        }
+    }
+
     let dom = cfg.domain.to_lowercase();
     if !dom.starts_with("http://") && !dom.starts_with("https://") {
         err!(