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/borderBottomWidth.js

@@ -0,0 +1,16 @@
+'use strict';
+
+var isValid = (module.exports.isValid = require('./borderWidth').isValid);
+
+module.exports.definition = {
+  set: function(v) {
+    if (isValid(v)) {
+      this._setProperty('border-bottom-width', v);
+    }
+  },
+  get: function() {
+    return this.getPropertyValue('border-bottom-width');
+  },
+  enumerable: true,
+  configurable: true,
+};