Log unsuccessful trade attempts in the db (#224)

* Save trade error in table trades

* Add one migration, Add error column to trades table

* changes in plugins.js file

* Apply standardjs to plugins.js file

migrations/1542811343367-add-error-to-trades.js

@@ -0,0 +1,15 @@
+var db = require('./db')
+
+exports.up = function (next) {
+  const sql = [
+    'alter table trades add column error text',
+  ]
+
+  db.multi(sql, next)
+}
+
+exports.down = function (next) {
+  const sql = ['alter table trades drop column error']
+
+  db.multi(sql, next)
+}