Fixed skipping of interfaces that don't have IPv6 in -6 mode

.gitignore

@@ -0,0 +1 @@
+/target

src/main.rs

@@ -95,11 +95,13 @@ impl Monitor {
 
     /// Was the file modified since the last check?
     fn check_file_modified(&mut self, lease_file_path: &str) -> bool {
-        let metadata = fs::metadata(&lease_file_path);
+        let metadata = match fs::metadata(&lease_file_path) {
+            Ok(m) => m,
+            Err(_) => return false,
+        };
         let current_timestamp = metadata
-            .expect("Unsupported platform")
             .modified()
-            .expect("Error getting modification timestamp");
+            .expect("Unsupported platform: cannot read file mtime");
 
         let last_timestamp = self
             .timestamps