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/_hasUnicodeWord.js

@@ -0,0 +1,15 @@
+/** Used to detect strings that need a more robust regexp to match words. */
+var reHasUnicodeWord = /[a-z][A-Z]|[A-Z]{2}[a-z]|[0-9][a-zA-Z]|[a-zA-Z][0-9]|[^a-zA-Z0-9 ]/;
+
+/**
+ * Checks if `string` contains a word composed of Unicode symbols.
+ *
+ * @private
+ * @param {string} string The string to inspect.
+ * @returns {boolean} Returns `true` if a word is found, else `false`.
+ */
+function hasUnicodeWord(string) {
+  return reHasUnicodeWord.test(string);
+}
+
+module.exports = hasUnicodeWord;