[feat] send json type content when call ctx.throw

[yunnysunny]

Describe the feature

Some time we want our server always reponse with json type, even if it throw custom error, for such code:

ctx.throw(403, 'need right access', {
      message: JSON.stringify({code: 1234, msg: 'you dont have right permission', userId: 'xxx'}),
      expose: true
})

But koa reponse all errors with text type, we can not change it.

Checklist

• [x] I have searched through GitHub issues for similar issues.
• [x] I have completely read through the README and documentation.

[siakc]

Guess you can put a middle there to catch the error and format the response before Koa. You can pass an object to ctx.throw() as any one of the arguments except first.