Fixed listing all themes when a theme path is invalid

- in the event one of the theme paths is invalid (a symlink that goes to
  a non-existent path), Ghost currently bails out of listing all themes
  because `fs.stat` throws an error
- in that case, we should just ignore the theme and continue
- this helps us prepare new symlinks whilst their source is not valid
- this commit wraps the code with try-catch to protect against that
- also adds a test

ghost/package-json/lib/package-json.js

@@ -132,8 +132,13 @@ async function readPackages(packagePath) {
         }
 
         if (file.isSymbolicLink()) {
-            const packageFileOrig = await fs.stat(join(packagePath, file.name));
-            return packageFileOrig.isDirectory();
+            try {
+                const packageFileOrig = await fs.stat(join(packagePath, file.name));
+                return packageFileOrig.isDirectory();
+            } catch (err) {
+                // if there's an error reading the symlink, we should just return false
+                return false;
+            }
         }
 
         // Check the remaining items to ensure they are a directory

ghost/package-json/test/read.test.js

@@ -137,6 +137,34 @@ describe('package-json read', function () {
                 .catch(done)
                 .finally(packagePath.removeCallback);
         });
+
+        it('should read directory and ignore invalid symlinks', async function () {
+            const packagePath = tmp.dirSync({unsafeCleanup: true});
+            const pkgJson = JSON.stringify({
+                name: 'test'
+            });
+
+            // create example theme
+            fs.mkdirSync(join(packagePath.name, 'testtheme'));
+            fs.writeFileSync(join(packagePath.name, 'testtheme', 'package.json'), pkgJson);
+            fs.writeFileSync(join(packagePath.name, 'testtheme', 'index.hbs'), '');
+
+            // Create a symlink that has a missing source dir
+            fs.symlinkSync(join(packagePath.name, 'missing-dir'), join(packagePath.name, 'source'));
+
+            try {
+                const pkgs = await packageJSON.readPackages(packagePath.name);
+                pkgs.should.eql({
+                    testtheme: {
+                        name: 'testtheme',
+                        path: join(packagePath.name, 'testtheme'),
+                        'package.json': null
+                    }
+                });
+            } finally {
+                await packagePath.removeCallback();
+            }
+        });
     });
 
     describe('readPackage', function () {