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/initial.js

@@ -0,0 +1,22 @@
+var baseSlice = require('./_baseSlice');
+
+/**
+ * Gets all but the last element of `array`.
+ *
+ * @static
+ * @memberOf _
+ * @since 0.1.0
+ * @category Array
+ * @param {Array} array The array to query.
+ * @returns {Array} Returns the slice of `array`.
+ * @example
+ *
+ * _.initial([1, 2, 3]);
+ * // => [1, 2]
+ */
+function initial(array) {
+  var length = array == null ? 0 : array.length;
+  return length ? baseSlice(array, 0, -1) : [];
+}
+
+module.exports = initial;