fix insufficient funds error msg & tests

lib/server.js

@@ -427,8 +427,9 @@ CopayServer.prototype.createTx = function(opts, cb) {
       });
 
       txp.inputs = self._selectUtxos(txp, utxos);
-      if (!txp.inputs)
-        return cb('insufficient funds')
+      if (!txp.inputs) {
+        return cb(new CopayError('INSUFFICIENTFUNDS', 'Insufficient funds'));
+      }
 
       // no need to do this now: // TODO remove this comment
       //self._createRawTx(txp);

test/integration.js

@@ -778,7 +778,8 @@ describe('Copay server', function() {
         };
 
         server.createTx(txOpts, function(err, tx) {
-          err.should.contain('insufficient');
+          err.code.should.equal('INSUFFICIENTFUNDS');
+          err.message.should.equal('Insufficient funds');
           server.getPendingTxs({
             walletId: '123'
           }, function(err, txs) {
@@ -879,7 +880,8 @@ describe('Copay server', function() {
             requestSignature: 'dummy',
           };
           server.createTx(txOpts2, function(err, tx) {
-            err.should.contain('insufficient');
+            err.code.should.equal('INSUFFICIENTFUNDS');
+            err.message.should.equal('Insufficient funds');
             should.not.exist(tx);
             server.getPendingTxs({
               walletId: '123'