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/fp/_falseOptions.js

@@ -0,0 +1,7 @@
+module.exports = {
+  'cap': false,
+  'curry': false,
+  'fixed': false,
+  'immutable': false,
+  'rearg': false
+};