Add check if maker has enough money

Previously if there wasnt enough money then the ProofOfFunding stage
would fail, however at that point the taker has already wasted miner
fees getting transactions confirmed, so it would be very nice to
let them know before.

src/maker_protocol.rs

@@ -468,14 +468,22 @@ fn handle_sign_senders_contract_tx(
         funding_txids.push(txinfo.senders_contract_tx.input[0].previous_output.txid);
         total_amount += txinfo.funding_input_value;
     }
-    log::info!(
-        "requested contracts amount={}, for funding txids = {:?}",
-        Amount::from_sat(total_amount),
-        funding_txids
-    );
-    Ok(Some(MakerToTakerMessage::SendersContractSig(
-        SendersContractSig { sigs },
-    )))
+    if total_amount < wallet.read().unwrap().get_offer_maxsize_cache() {
+        log::info!(
+            "requested contracts amount={}, for funding txids = {:?}",
+            Amount::from_sat(total_amount),
+            funding_txids
+        );
+        Ok(Some(MakerToTakerMessage::SendersContractSig(
+            SendersContractSig { sigs },
+        )))
+    } else {
+        log::info!(
+            "rejecting contracts for amount={} because not enough funds",
+            Amount::from_sat(total_amount)
+        );
+        Err(Error::Protocol("not enough funds"))
+    }
 }
 
 fn handle_proof_of_funding(