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/load-json-file/index.js

@@ -0,0 +1,11 @@
+'use strict';
+const path = require('path');
+const fs = require('graceful-fs');
+const stripBom = require('strip-bom');
+const parseJson = require('parse-json');
+const pify = require('pify');
+
+const parse = (data, fp) => parseJson(stripBom(data), path.relative('.', fp));
+
+module.exports = fp => pify(fs.readFile)(fp, 'utf8').then(data => parse(data, fp));
+module.exports.sync = fp => parse(fs.readFileSync(fp, 'utf8'), fp);