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/node_modules/extend-shallow/index.js

@@ -0,0 +1,33 @@
+'use strict';
+
+var isObject = require('is-extendable');
+
+module.exports = function extend(o/*, objects*/) {
+  if (!isObject(o)) { o = {}; }
+
+  var len = arguments.length;
+  for (var i = 1; i < len; i++) {
+    var obj = arguments[i];
+
+    if (isObject(obj)) {
+      assign(o, obj);
+    }
+  }
+  return o;
+};
+
+function assign(a, b) {
+  for (var key in b) {
+    if (hasOwn(b, key)) {
+      a[key] = b[key];
+    }
+  }
+}
+
+/**
+ * Returns true if the given `key` is an own property of `obj`.
+ */
+
+function hasOwn(obj, key) {
+  return Object.prototype.hasOwnProperty.call(obj, key);
+}