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/_castPath.js

@@ -0,0 +1,21 @@
+var isArray = require('./isArray'),
+    isKey = require('./_isKey'),
+    stringToPath = require('./_stringToPath'),
+    toString = require('./toString');
+
+/**
+ * Casts `value` to a path array if it's not one.
+ *
+ * @private
+ * @param {*} value The value to inspect.
+ * @param {Object} [object] The object to query keys on.
+ * @returns {Array} Returns the cast property path array.
+ */
+function castPath(value, object) {
+  if (isArray(value)) {
+    return value;
+  }
+  return isKey(value, object) ? [value] : stringToPath(toString(value));
+}
+
+module.exports = castPath;