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/_reEscape.js

@@ -0,0 +1,4 @@
+/** Used to match template delimiters. */
+var reEscape = /<%-([\s\S]+?)%>/g;
+
+module.exports = reEscape;