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/_asciiWords.js

@@ -0,0 +1,15 @@
+/** Used to match words composed of alphanumeric characters. */
+var reAsciiWord = /[^\x00-\x2f\x3a-\x40\x5b-\x60\x7b-\x7f]+/g;
+
+/**
+ * Splits an ASCII `string` into an array of its words.
+ *
+ * @private
+ * @param {string} The string to inspect.
+ * @returns {Array} Returns the words of `string`.
+ */
+function asciiWords(string) {
+  return string.match(reAsciiWord) || [];
+}
+
+module.exports = asciiWords;