Allow to customize error handler

[sm2017]

Checklist

• [x] npm test passes
• [x] tests and/or benchmarks are included
• [ ] documentation is changed or added
• [x] commit message follows commit guidelines

Description of change

As you always catch errors it is impossible to customizing errors for instance it is not possible to have a JSON response where we have UnauthorizedRequestError , or we cannot prevent ctx.app.emit('error', e, ctx); emitting , it is not good to log UnauthorizedRequestError or UnauthorizedClientError in production mode

[codecov-io]

Codecov Report

Merging #42 into master will decrease coverage by 0.68%. The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master      #42      +/-   ##
==========================================
- Coverage      80%   79.31%   -0.69%     
==========================================
  Files           3        3              
  Lines          85       87       +2     
==========================================
+ Hits           68       69       +1     
- Misses         17       18       +1

Impacted Files	Coverage Δ
config/config.default.js	100% <ø> (ø)	:arrow_up:
lib/server.js	79.41% <66.66%> (-0.9%)	:arrow_down:

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[Azard]

@sm2017 Could you add a test case for this feature ?

[sm2017]

Added Do you have a error handler I expect it throws when we have no middleware to handle error

[Azard]

@sm2017 Its better to handle the fail test case, or just remove the test case

[sm2017]

@azard can you add test case? I expect 500 error, because it must throw unhanded exception but may be there is a middleware that catch it

[Azard]

https://github.com/Azard/egg-oauth2-server/pull/44