Throw error if tc interface retrieval fails (#56)

* Throw error if tc interface retrieval fails

On Linux if `net-tools` are not installed `tc` will fail to get the default interface using the `route` command. Or it might fail for any other reason.

This commit adds a safeguard to bug out on err and displays the actual stderr received instead of hiding it.

lib/tc.js

@@ -7,6 +7,13 @@ async function getDefaultInterface() {
     "sudo route | grep '^default' | grep -o '[^ ]*$' | tr -d '\n'",
     { shell: true }
   );
+
+  if (result.stdout.length === 0 && result.stderr.length > 0) {
+    throw new Error(
+      'There was an error getting the default interface:\n\n' + result.stderr
+    );
+  }
+
   return result.stdout;
 }