Fix missing object wrapper for errors in pino log

[ofavre]

log is a pino logger, the first argument is a mapping, so in order to give it an error object, we must give it an object with an err field, otherwise the error object itself is dissected as if it were a mapping.

This commit fixes places where this has been forgotten.
I only changed source files, not test files because it's not a big deal there.

Checklist

• run npm run test and npm run benchmark
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows Code of conduct

npm run benchmark exits this way on my machine, on my branch just like on master:

[0] {"level":30,"time":1537595975197,"msg":"request completed","pid":30338,"hostname":"Ofavre-Home-T5500","reqId":38361,"res":{"statusCode":200},"responseTime":0.18976199626922607,"v":1}
[0] {"level":50,"time":1537595975197,"msg":"client error","pid":30338,"hostname":"Ofavre-Home-T5500","type":"Error","stack":"Error: write EPIPE\n    at WriteWrap.afterWrite [as oncomplete] (net.js:835:14)","errno":"EPIPE","code":"EPIPE","syscall":"write","v":1}
[0] {"level":50,"time":1537595975198,"msg":"client error","pid":30338,"hostname":"Ofavre-Home-T5500","type":"Error","stack":"Error: read ECONNRESET\n    at TCP.onread (net.js:659:25)","errno":"ECONNRESET","code":"ECONNRESET","syscall":"read","v":1}
[0] node ./examples/simple.js exited with code SIGTERM

Benchmarking such a change makes no sense anyway.

[mcollina]

LGTM.

log.error(err) is actually valid in pino, but it's less configurable.

[cemremengu]

LGTM

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