be more precise on error message

lib/server.js

@@ -479,7 +479,7 @@ WalletService.prototype._selectTxInputs = function(txp, cb) {
 
     var txMinAmount = txp.amount + Bitcore.Transaction.FEE_PER_KB;
     if (balance.totalAmount < txMinAmount)
-      return cb(new ClientError('INSUFFICIENTFUNDS', 'Insufficient funds'));
+      return cb(new ClientError('INSUFFICIENTFUNDS', 'Insufficient funds' + (balance.totalAmount >= txp.amount ? ' for fee' : '')));
 
     if ((balance.totalAmount - balance.lockedAmount) < txMinAmount)
       return cb(new ClientError('LOCKEDFUNDS', 'Funds are locked by pending transaction proposals'));

test/integration/server.js

@@ -938,7 +938,7 @@ describe('Copay server', function() {
         server.createTx(txOpts, function(err, tx) {
           should.exist(err);
           err.code.should.equal('INSUFFICIENTFUNDS');
-          err.message.should.equal('Insufficient funds');
+          err.message.should.equal('Insufficient funds for fee');
           done();
         });
       });