feat: Support get context in error formatter

[axetroy]

In some cases, I need get context in error formatter.

example:

Translate error message by context.header,

If accept-language is en-US, then output English error message
if accept-language is zh-CN, then output Chiness error message

[axetroy]

@chentsulin
Hey guy, can you consider merger this PR?
It's useful in case.
If something go wrong, please tell me and I will fix it.
If you don't consider this feature, I will publish antoher npm package, At least let me know.

Thanks for you time.

[chentsulin]

@axetroy Thanks! Released as v0.7.4
https://github.com/chentsulin/koa-graphql/releases/tag/v0.7.4