fix: pass middleware errors to pino-http

[yowainwright]

Description

This revives the small fix from #20 against the current codebase.

When downstream Koa middleware throws:

1. koa-pino-logger currently logs the error immediately with ctx.log.error({ err }), then
2. rethrows. pino-http also logs the request on response completion, which
3. can produce duplicate request logs for the same failure.

This change stores the middleware error on ctx.res.err and rethrows it, letting pino-http include the error in its normal response-completion log.

• Fixes using ctx.throw results in both request errored and request completed being logged #19.
• Supersedes interop with pino-http by setting res.err instead of calling log.error #20.

Validation

• npm test

Thank you!
~Jeff