wrap node-sass error message in Error object

closes #15 - when using Metalsmith cli, errors are
expected to an actual error object, not a string

Author: stevenschobert

lib/index.js

@@ -32,15 +32,18 @@
             includePaths: includes,
             imagePath: imagePath,
             outputStyle: outputStyle,
-            success: function(css) {
+            success: function handleSuccess(css) {
               // replace contents
               file.contents = new Buffer(css);
               // rename file extension
               files[path.join(outputDir, path.basename(filename).replace('.scss', '.css'))] = file;
               delete files[filename];
               done();
             },
-            error: function(err) {
+            error: function handleError(err) {
+              if (typeof err === 'string') {
+                err = new Error(err);
+              }
               done(err);
             }
           });

test/test.js

@@ -105,15 +105,15 @@
         });
       });
 
-      it('should fail when invalid file provided', function(done) {
+      it('should correctly report errors to Metalsmith', function(done) {
         metalsmith(__dirname)
           .source('fixtures/invalid/src')
           .destination('fixtures/invalid/build')
           .use(sass({
             outputStyle: 'expanded'
           }))
           .build(function (err) {
-            assert.notEqual(err, null, "Error should be set.");
+            assert(err.message && /aninvalidrule/.test(err.message));
             assert(!exists(join(__dirname, 'fixtures/invalid/build/invalid.scss')));
             done();
           });