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/_baseZipObject.js

@@ -0,0 +1,23 @@
+/**
+ * This base implementation of `_.zipObject` which assigns values using `assignFunc`.
+ *
+ * @private
+ * @param {Array} props The property identifiers.
+ * @param {Array} values The property values.
+ * @param {Function} assignFunc The function to assign values.
+ * @returns {Object} Returns the new object.
+ */
+function baseZipObject(props, values, assignFunc) {
+  var index = -1,
+      length = props.length,
+      valsLength = values.length,
+      result = {};
+
+  while (++index < length) {
+    var value = index < valsLength ? values[index] : undefined;
+    assignFunc(result, props[index], value);
+  }
+  return result;
+}
+
+module.exports = baseZipObject;