add docs for error ctx

docs/api.md

@@ -538,6 +538,14 @@ throw err;
   By default outputs all errors to stderr unless __NODE_ENV__ is "test". To perform custom error-handling logic such as centralized logging you
   can add an "error" event listener:
 
+```js
+app.on('error', function(err){
+  log.error('server error', err);
+});
+```
+
+  If an error in the req/res cycle and it is _not_ possible to respond to the client, the `Context` instance is also passed:
+
 ```js
 app.on('error', function(err){
   log.error('server error', err);

lib/context.js

@@ -1,3 +1,4 @@
+
 /**
  * Module dependencies.
  */