Improve plugin loading to not fake module errors

lib/plugins/loadPlugin.js

@@ -32,6 +32,7 @@ function loadPlugin(book, plugin) {
     var p = Promise()
     .then(function() {
         var packageContent;
+        var packageMain;
         var content;
 
         // Locate plugin and load package.json
@@ -49,15 +50,20 @@ function loadPlugin(book, plugin) {
             return;
         }
 
-        // Load plugin JS content
+        // Locate the main package
         try {
-            content = require(pkgPath);
-        } catch(err) {
-            // It's no big deal if the plugin doesn't have an "index.js"
-            // (For example: themes)
-            if (isModuleNotFound(err)) {
-                content = {};
-            } else {
+            var indexJs = path.normalize(packageContent.main || 'index.js');
+            packageMain = resolve.sync('./' + indexJs, { basedir: pkgPath });
+        } catch (err) {
+            if (!isModuleNotFound(err)) throw err;
+            packageMain = undefined;
+        }
+
+        // Load plugin JS content
+        if (packageMain) {
+            try {
+                content = require(packageMain);
+            } catch(err) {
                 throw new error.PluginError(err, {
                     plugin: name
                 });