test: coverage 100%

[climba03003]

Changes:

1. fix unhandled error throw by plugin
2. increase test coverage to 100%
3. prepare for tap 15

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 the Developer's Certification of Origin
  and the Code of conduct

[climba03003]

Throwing error here will lead to avvio timeout as the throwing error is not properly consumed by avvio.
We need to pass the Error to the callback if we are using callback-style here.

[mcollina]

lgtm