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/json5/lib/index.js

@@ -0,0 +1,9 @@
+const parse = require('./parse')
+const stringify = require('./stringify')
+
+const JSON5 = {
+    parse,
+    stringify,
+}
+
+module.exports = JSON5