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/path-exists/index.js

@@ -0,0 +1,17 @@
+'use strict';
+const fs = require('fs');
+
+module.exports = fp => new Promise(resolve => {
+	fs.access(fp, err => {
+		resolve(!err);
+	});
+});
+
+module.exports.sync = fp => {
+	try {
+		fs.accessSync(fp);
+		return true;
+	} catch (err) {
+		return false;
+	}
+};