add error handling example

examples/errors.js

@@ -0,0 +1,42 @@
+
+var http = require('http');
+var koa = require('..');
+var app = koa();
+
+// look ma, error propagation!
+
+app.use(function(next){
+  return function *(){
+    try {
+      yield next;
+    } catch (err) {
+      // some errors will have .status
+      // however this is not a guarantee
+      this.status = err.status || 500;
+      this.type = 'html';
+      this.body = '<p>Something <em>exploded</em>, please contact Maru.</p>';
+    
+      // since we handled this manually we'll
+      // want to delegate to the regular app
+      // level error handling as well so that
+      // centralized still functions correctly.
+      this.app.emit('error', err);
+    }
+  }
+});
+
+// response
+
+app.use(function(next){
+  return function *(){
+    throw new Error('boom boom');
+  }
+});
+
+// error handler
+
+app.on('error', function(err){
+  console.log('sent error %s to the cloud', err.message);
+});
+
+app.listen(3000);