Fix error response for adapter_failure

[miike]

Describe the bug
A clear and concise description of what the bug is.

In certain instances a Scala object (AdapterFailure) is returned rather than the expected bad rows JSON.

To Reproduce
Steps to reproduce the behavior or code snippets that produce the issue.

1. Spin up Micro
2. curl -X GET http://localhost:9090/i
3. curl -X GET http://localhost:9090/micro/bad

Expected behavior
A clear and concise description of what you expected to happen.

The bad JSON response should contain an error response that contains a failed event with iglu:com.snowplowanalytics.snowplow.badrows/adapter_failures/jsonschema/1-0-0.

Screenshots

Instead a AdapterFailures is returned as a string as part of the JSON payload - rather than the failed event JSON.

"errors":["Error while extracting event(s) from collector payload and validating it/them.","AdapterFailures(Processor(snowplow-micro,1.1.3-rc4.dev.1),AdapterFailures(2021-08-21T04:11:49.049128Z,com.snowplowanalytics.snowplow,tp1,NonEmptyList(InputData(querystring,None,empty querystring: not a valid URI redirect))),CollectorPayload(com.snowplowanalytics.snowplow,tp1,List(),None,None,ssc-2.3.0-stdout$,UTF-8,Some(localhost),Some(2021-08-21T04:11:49.014Z),Some(172.17.0.1),Some(Mozilla/5.0),None,List(Timeout-Access: <function1>, Host: localhost:9090, Connection: keep-alive, s...),Some(041a959b-63d1-4eb4-a851-83833e407470)))"]}]

[adatzer]

Hello @miike and thank you for raising this issue!

Through it, we've been thinking about how Micro's bad row output could be improved in general.
So, even though we will address this present issue in the coming release, it can be considered as only a partial fix to that broader issue (which is probably for a v2 release as a breaking API change).
It would be great if you could share your ideas about it or any particular use cases you have in mind!