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/yargs/lib/yerror.js

@@ -0,0 +1,11 @@
+'use strict'
+function YError (msg) {
+  this.name = 'YError'
+  this.message = msg || 'yargs error'
+  Error.captureStackTrace(this, YError)
+}
+
+YError.prototype = Object.create(Error.prototype)
+YError.prototype.constructor = YError
+
+module.exports = YError