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/_util.js

@@ -0,0 +1,16 @@
+module.exports = {
+  'ary': require('../ary'),
+  'assign': require('../_baseAssign'),
+  'clone': require('../clone'),
+  'curry': require('../curry'),
+  'forEach': require('../_arrayEach'),
+  'isArray': require('../isArray'),
+  'isError': require('../isError'),
+  'isFunction': require('../isFunction'),
+  'isWeakMap': require('../isWeakMap'),
+  'iteratee': require('../iteratee'),
+  'keys': require('../_baseKeys'),
+  'rearg': require('../rearg'),
+  'toInteger': require('../toInteger'),
+  'toPath': require('../toPath')
+};