Fix loadFile error handling in sync mode

When running in sync mode (syncImport option), loadFile should call the
callback instead of returning a promise. This change implements logic
similar to [1].

[1] https://github.com/less/less.js/blob/49cbe520f6b38b4b8797dcba491501c0c461c7f5/lib/less-node/file-manager.js#L31

lib/npm-file-manager.js

@@ -48,10 +48,15 @@ module.exports = function(less) {
     };
 
     NpmFileManager.prototype.loadFile = function(filename, currentDirectory, options, environment, callback) {
+        options = options || {};
         try {
             filename = this.resolve(filename, currentDirectory);
         }
         catch(e) {
+            if (options.syncImport) {
+                callback(e);
+                return;
+            }
             return new PromiseConstructor(
                 function(fullfill, reject) {
                     reject(e);