[breaking change] don't bind onerror to context

[dead-horse]

1. People should understand javascript's this, and it's easy to handle with arrow functions just like stream.on('error', err => this.onerror(err));.
2. We already handled all the errors automatically in koa, 99.9% people won't use this.onerror in app level. We have written 100+ apps and middlewares in production, no one used this.onerror.
3. It's has higher performance(~10% increase).

• before

  1 middleware
  9134.88

  5 middleware
  9209.45

  10 middleware
  8261.05

  15 middleware
  8873.18

  20 middleware
  8957.63

  30 middleware
  9036.78

  50 middleware
  8543.95

  100 middleware
  8088.44

• after

  1 middleware
  9939.28

  5 middleware
  9806.14

  10 middleware
  9636.60

  15 middleware
  9603.57

  20 middleware
  9664.59

  30 middleware
  9695.19

  50 middleware
  9220.74

  100 middleware
  8498.86

[codecov-io]

Current coverage is 100% (diff: 100%)

Merging #800 into v2.x will not change coverage

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  Branches    100    100          
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  Hits        415    415          
  Misses        0      0          
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[dead-horse]

@koajs any thoughts?

[tj]

👍 kinda weird that it's pre-bound, onerror is private anyway so it should be fine

[fengmk2]

+1 for the considerable performance increase.

[fengmk2]

Create onerror handler once should be better.

const onerror = err => ctx.onerror(err);
onFinished(res, onerror);
fn(ctx).then(() => respond(ctx)).catch(onerror);

[juliangruber]

👍