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/_stringSize.js

@@ -0,0 +1,18 @@
+var asciiSize = require('./_asciiSize'),
+    hasUnicode = require('./_hasUnicode'),
+    unicodeSize = require('./_unicodeSize');
+
+/**
+ * Gets the number of symbols in `string`.
+ *
+ * @private
+ * @param {string} string The string to inspect.
+ * @returns {number} Returns the string size.
+ */
+function stringSize(string) {
+  return hasUnicode(string)
+    ? unicodeSize(string)
+    : asciiSize(string);
+}
+
+module.exports = stringSize;