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/cssstyle/lib/properties/bottom.js

@@ -0,0 +1,14 @@
+'use strict';
+
+var parseMeasurement = require('../parsers').parseMeasurement;
+
+module.exports.definition = {
+  set: function(v) {
+    this._setProperty('bottom', parseMeasurement(v));
+  },
+  get: function() {
+    return this.getPropertyValue('bottom');
+  },
+  enumerable: true,
+  configurable: true,
+};