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/har-schema/lib/index.js

@@ -0,0 +1,22 @@
+'use strict'
+
+module.exports = {
+  afterRequest: require('./afterRequest.json'),
+  beforeRequest: require('./beforeRequest.json'),
+  browser: require('./browser.json'),
+  cache: require('./cache.json'),
+  content: require('./content.json'),
+  cookie: require('./cookie.json'),
+  creator: require('./creator.json'),
+  entry: require('./entry.json'),
+  har: require('./har.json'),
+  header: require('./header.json'),
+  log: require('./log.json'),
+  page: require('./page.json'),
+  pageTimings: require('./pageTimings.json'),
+  postData: require('./postData.json'),
+  query: require('./query.json'),
+  request: require('./request.json'),
+  response: require('./response.json'),
+  timings: require('./timings.json')
+}