support errors and update docs

docs/api/context.md

@@ -102,10 +102,11 @@ throw err;
   error messages since you do not want to leak failure
   details.
 
-  You may optionall pass a `properties` object which is merged into the error as-is, useful for decorating machine-friendly errors which are reported to the requester upstream.
+  You may optionally pass a `properties` object which is merged into the error as-is, useful for decorating machine-friendly errors which are reported to the requester upstream.
 
 ```js
 this.throw(401, 'access_denied', { user: user });
+this.throw('access_denied', { user: user });
 ```
 
 ### ctx.respond

test/context/throw.js

@@ -191,3 +191,19 @@ describe('ctx.throw(status, props)', function(){
     }
   })
 })
+
+describe('ctx.throw(err, props)', function(){
+  it('should mixin props', function(done){
+    var ctx = context();
+
+    try {
+      ctx.throw(new Error('test'), { prop: true });
+    } catch (err) {
+      assert('test' == err.message);
+      assert(500 == err.status);
+      assert(false === err.expose);
+      assert(true === err.prop);
+      done();
+    }
+  })
+})