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/_toKey.js

@@ -0,0 +1,21 @@
+var isSymbol = require('./isSymbol');
+
+/** Used as references for various `Number` constants. */
+var INFINITY = 1 / 0;
+
+/**
+ * Converts `value` to a string key if it's not a string or symbol.
+ *
+ * @private
+ * @param {*} value The value to inspect.
+ * @returns {string|symbol} Returns the key.
+ */
+function toKey(value) {
+  if (typeof value == 'string' || isSymbol(value)) {
+    return value;
+  }
+  var result = (value + '');
+  return (result == '0' && (1 / value) == -INFINITY) ? '-0' : result;
+}
+
+module.exports = toKey;