Added checks to make sure the specified track exists

Added precondition checks to validate input files and paths to return more informative error messages as per #2

node_modules/ajv/lib/compile/schema_obj.js

@@ -0,0 +1,9 @@
+'use strict';
+
+var util = require('./util');
+
+module.exports = SchemaObject;
+
+function SchemaObject(obj) {
+  util.copy(obj, this);
+}