Fixed incorrect example of ctx.onerror usage

[PaulAnnekov]

https://koajs.com/#stream provides the following example on how to handle errors in koa handler:

const PassThrough = require('stream').PassThrough;

app.use(async ctx => {
  ctx.body = someHTTPStream.on('error', ctx.onerror).pipe(PassThrough());
});

but this example is buggy. Consider the following code:

const request = require('request');
const { PassThrough } = require('stream');
const Koa = require('koa');
const route = require('koa-route');

function getFile(ctx) {
    ctx.body = request
        .get('http://asdasd.com/')
        .on('error', ctx.onerror)
        .pipe(new PassThrough());
}

const init = () => {
    const app = new Koa();

    app.use(route.get(`/get`, getFile));

    app.listen(12345);
};

init();

If you will execute curl http://localhost:12345/get it will result in exception:

/home/node/app/node_modules/koa/lib/context.js:117
    this.app.emit('error', err, this);
             ^

TypeError: Cannot read property 'emit' of undefined
    at Request.onerror (/home/node/app/node_modules/koa/lib/context.js:117:14)
    at Request.emit (events.js:203:15)
    at Request.onRequestError (/home/node/app/node_modules/request/request.js:877:8)
    at ClientRequest.emit (events.js:198:13)
    at Socket.socketErrorListener (_http_client.js:392:9)
    at Socket.emit (events.js:198:13)
    at emitErrorNT (internal/streams/destroy.js:91:8)
    at emitErrorAndCloseNT (internal/streams/destroy.js:59:3)
    at process._tickCallback (internal/process/next_tick.js:63:19)

That's because .on('error', ctx.onerror) will call onerror with different context, not ctx. That context doesn't have app property (

koa/lib/context.js

Line 121 in 8d52105

this.app.emit('error', err, this);

), which results in exception.

I have updated docs to preserve context.

[codecov]

Codecov Report

Merging #1459 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1459   +/-   ##
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  Coverage   99.38%   99.38%           
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  Files           4        4           
  Lines         484      484           
  Branches      130      130           
=======================================
  Hits          481      481           
  Partials        3        3           

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