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/lineHeight.js

@@ -0,0 +1,26 @@
+'use strict';
+
+var TYPES = require('../parsers').TYPES;
+var valueType = require('../parsers').valueType;
+
+module.exports.isValid = function isValid(v) {
+  var type = valueType(v);
+  return (
+    (type === TYPES.KEYWORD && v.toLowerCase() === 'normal') ||
+    v.toLowerCase() === 'inherit' ||
+    type === TYPES.NUMBER ||
+    type === TYPES.LENGTH ||
+    type === TYPES.PERCENT
+  );
+};
+
+module.exports.definition = {
+  set: function(v) {
+    this._setProperty('line-height', v);
+  },
+  get: function() {
+    return this.getPropertyValue('line-height');
+  },
+  enumerable: true,
+  configurable: true,
+};