Improve error handling

Arguments are now passed to an command error in case they exist
An error is only emitted if that very same error is not already handled in a callback

lib/customError.js

@@ -0,0 +1,16 @@
+'use strict';
+
+var util = require('util');
+
+function CommandError (error) {
+    Error.captureStackTrace(this, this.constructor);
+    this.name = this.constructor.name;
+    this.message = error.message;
+    for (var keys = Object.keys(error), key = keys.pop(); key; key = keys.pop()) {
+        this[key] = error[key];
+    }
+}
+
+util.inherits(CommandError, Error);
+
+module.exports = CommandError;