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/_baseSample.js

@@ -0,0 +1,15 @@
+var arraySample = require('./_arraySample'),
+    values = require('./values');
+
+/**
+ * The base implementation of `_.sample`.
+ *
+ * @private
+ * @param {Array|Object} collection The collection to sample.
+ * @returns {*} Returns the random element.
+ */
+function baseSample(collection) {
+  return arraySample(values(collection));
+}
+
+module.exports = baseSample;