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/snapdragon/lib/position.js

@@ -0,0 +1,14 @@
+'use strict';
+
+var define = require('define-property');
+
+/**
+ * Store position for a node
+ */
+
+module.exports = function Position(start, parser) {
+  this.start = start;
+  this.end = { line: parser.line, column: parser.column };
+  define(this, 'content', parser.orig);
+  define(this, 'source', parser.options.source);
+};