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/lodash/_matchesStrictComparable.js

@@ -0,0 +1,20 @@
+/**
+ * A specialized version of `matchesProperty` for source values suitable
+ * for strict equality comparisons, i.e. `===`.
+ *
+ * @private
+ * @param {string} key The key of the property to get.
+ * @param {*} srcValue The value to match.
+ * @returns {Function} Returns the new spec function.
+ */
+function matchesStrictComparable(key, srcValue) {
+  return function(object) {
+    if (object == null) {
+      return false;
+    }
+    return object[key] === srcValue &&
+      (srcValue !== undefined || (key in Object(object)));
+  };
+}
+
+module.exports = matchesStrictComparable;