refactor(example): early error if RPC_URL isn't set in price_printer

examples/price_printer/main.rs

@@ -43,6 +43,11 @@ async fn main() {
     let tycho_api_key: String =
         env::var("TYCHO_API_KEY").unwrap_or_else(|_| "sampletoken".to_string());
 
+    // Perform an early check to ensure `RPC_URL` is set.
+    // This prevents errors from occurring later during UI interactions.
+    // Can be commented out if only using the example with uniswap_v2 and uniswap_v3.
+    env::var("RPC_URL").expect("RPC_URL env variable should be set");
+
     // Create communication channels for inter-thread communication
     let (tick_tx, tick_rx) = mpsc::channel::<BlockUpdate>(12);