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/_lazyReverse.js

@@ -0,0 +1,23 @@
+var LazyWrapper = require('./_LazyWrapper');
+
+/**
+ * Reverses the direction of lazy iteration.
+ *
+ * @private
+ * @name reverse
+ * @memberOf LazyWrapper
+ * @returns {Object} Returns the new reversed `LazyWrapper` object.
+ */
+function lazyReverse() {
+  if (this.__filtered__) {
+    var result = new LazyWrapper(this);
+    result.__dir__ = -1;
+    result.__filtered__ = true;
+  } else {
+    result = this.clone();
+    result.__dir__ *= -1;
+  }
+  return result;
+}
+
+module.exports = lazyReverse;