Improve error messages when determining BTC amount to be swapped

swap/src/bin/swap_cli.rs

@@ -313,14 +313,18 @@ async fn determine_btc_to_swap(
             bid_quote.max_quantity
         );
 
-        let new_balance = wait_for_deposit.await?;
+        let new_balance = wait_for_deposit
+            .await
+            .context("Failed to wait for Bitcoin deposit")?;
 
         info!("Received {}", new_balance);
     } else {
         info!("Found {} in wallet", initial_balance);
     }
 
-    let max_giveable = max_giveable.await?;
+    let max_giveable = max_giveable
+        .await
+        .context("Failed to compute max 'giveable' Bitcoin amount")?;
     let max_accepted = bid_quote.max_quantity;
 
     if max_giveable > max_accepted {

swap/src/bitcoin/wallet.rs

@@ -155,7 +155,7 @@ impl Wallet {
         tx_builder.set_single_recipient(dummy_script);
         tx_builder.drain_wallet();
         tx_builder.fee_rate(self.select_feerate());
-        let (_, details) = tx_builder.finish()?;
+        let (_, details) = tx_builder.finish().context("Failed to build transaction")?;
 
         let max_giveable = details.sent - details.fees;