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/_wrapperClone.js

@@ -0,0 +1,23 @@
+var LazyWrapper = require('./_LazyWrapper'),
+    LodashWrapper = require('./_LodashWrapper'),
+    copyArray = require('./_copyArray');
+
+/**
+ * Creates a clone of `wrapper`.
+ *
+ * @private
+ * @param {Object} wrapper The wrapper to clone.
+ * @returns {Object} Returns the cloned wrapper.
+ */
+function wrapperClone(wrapper) {
+  if (wrapper instanceof LazyWrapper) {
+    return wrapper.clone();
+  }
+  var result = new LodashWrapper(wrapper.__wrapped__, wrapper.__chain__);
+  result.__actions__ = copyArray(wrapper.__actions__);
+  result.__index__  = wrapper.__index__;
+  result.__values__ = wrapper.__values__;
+  return result;
+}
+
+module.exports = wrapperClone;