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/number.js

@@ -0,0 +1,5 @@
+module.exports = {
+  'clamp': require('./clamp'),
+  'inRange': require('./inRange'),
+  'random': require('./random')
+};