better error if insufficient funds for user action (#1002)

pkg/debugapi/chequebook.go

@@ -22,6 +22,7 @@ var (
 	errChequebookNoAmount          = "did not specify amount"
 	errChequebookNoWithdraw        = "cannot withdraw"
 	errChequebookNoDeposit         = "cannot deposit"
+	errChequebookInsufficientFunds = "insufficient funds"
 	errCantLastChequePeer          = "cannot get last cheque for peer"
 	errCantLastCheque              = "cannot get last cheque for all peers"
 	errCannotCash                  = "cannot cash cheque"
@@ -298,6 +299,12 @@ func (s *server) chequebookWithdrawHandler(w http.ResponseWriter, r *http.Reques
 	}
 
 	txHash, err := s.Chequebook.Withdraw(r.Context(), amount)
+	if errors.Is(err, chequebook.ErrInsufficientFunds) {
+		jsonhttp.BadRequest(w, errChequebookInsufficientFunds)
+		s.Logger.Debugf("debug api: chequebook withdraw: %v", err)
+		s.Logger.Error("debug api: cannot withdraw from chequebook")
+		return
+	}
 	if err != nil {
 		jsonhttp.InternalServerError(w, errChequebookNoWithdraw)
 		s.Logger.Debugf("debug api: chequebook withdraw: %v", err)
@@ -324,6 +331,12 @@ func (s *server) chequebookDepositHandler(w http.ResponseWriter, r *http.Request
 	}
 
 	txHash, err := s.Chequebook.Deposit(r.Context(), amount)
+	if errors.Is(err, chequebook.ErrInsufficientFunds) {
+		jsonhttp.BadRequest(w, errChequebookInsufficientFunds)
+		s.Logger.Debugf("debug api: chequebook deposit: %v", err)
+		s.Logger.Error("debug api: cannot deposit from chequebook")
+		return
+	}
 	if err != nil {
 		jsonhttp.InternalServerError(w, errChequebookNoDeposit)
 		s.Logger.Debugf("debug api: chequebook deposit: %v", err)