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/_createCompounder.js

@@ -0,0 +1,24 @@
+var arrayReduce = require('./_arrayReduce'),
+    deburr = require('./deburr'),
+    words = require('./words');
+
+/** Used to compose unicode capture groups. */
+var rsApos = "['\u2019]";
+
+/** Used to match apostrophes. */
+var reApos = RegExp(rsApos, 'g');
+
+/**
+ * Creates a function like `_.camelCase`.
+ *
+ * @private
+ * @param {Function} callback The function to combine each word.
+ * @returns {Function} Returns the new compounder function.
+ */
+function createCompounder(callback) {
+  return function(string) {
+    return arrayReduce(words(deburr(string).replace(reApos, '')), callback, '');
+  };
+}
+
+module.exports = createCompounder;