fix(plugin-koa): Ensure unhandled Koa errors are logged out

[bengourley]

Our plugin adds an "onerror" handler to the Koa app, which stops the default from being called.

The default error handler which prints the error is available at ctx.app.onerror so by calling through to that after notifying, it exhibits the default behaviour.

Tested manually with notifyReleaseStages both allowing and preventing delivery and added a unit test to ensure the original handler is called.

Output before:

[bugsnag] Report not sent due to releaseStage/notifyReleaseStages configuration

Output after:

[bugsnag] Report not sent due to releaseStage/notifyReleaseStages configuration

  Error: Invalid DSL syntax
      at /Users/bengourley/Development/bugsnag-js/packages/plugin-koa/scratch/app.js:29:29
      at dispatch (/Users/bengourley/Development/bugsnag-js/packages/plugin-koa/scratch/node_modules/koa-router/node_modules/koa-compose/index.js:44:32)
      at next (/Users/bengourley/Development/bugsnag-js/packages/plugin-koa/scratch/node_modules/koa-router/node_modules/koa-compose/index.js:45:18)
      at /Users/bengourley/Development/bugsnag-js/packages/plugin-koa/scratch/node_modules/koa-router/lib/router.js:346:16
      at dispatch (/Users/bengourley/Development/bugsnag-js/packages/plugin-koa/scratch/node_modules/koa-router/node_modules/koa-compose/index.js:44:32)
      at /Users/bengourley/Development/bugsnag-js/packages/plugin-koa/scratch/node_modules/koa-router/node_modules/koa-compose/index.js:36:12
      at dispatch (/Users/bengourley/Development/bugsnag-js/packages/plugin-koa/scratch/node_modules/koa-router/lib/router.js:351:31)
      at dispatch (/Users/bengourley/Development/bugsnag-js/packages/plugin-koa/scratch/node_modules/koa-compose/index.js:42:32)
      at requestHandler (/Users/bengourley/Development/bugsnag-js/packages/plugin-koa/dist/bugsnag-koa.js:778:15)
      at dispatch (/Users/bengourley/Development/bugsnag-js/packages/plugin-koa/scratch/node_modules/koa-compose/index.js:42:32)

Fixes #602

[bugsnagbot]

@bugsnag/browser bundle size diff

	Minified	Minfied + Gzipped
Before	40.89 kB	12.24 kB
After	40.89 kB	12.24 kB
±	No change	No change

Generated by 🚫 dangerJS against 41120d3

[bengourley]

https://github.com/koajs/koa/blob/a245d18a131341feec4f87659746954e78cae780/lib/application.js#L183