Throw proper exception when request parsing fails

Currently calling the .body method (or injecting it) and sending invalid content to the endpoint throws a generic java.text.ParseException or MapperWrongTypeException from abstract-data. The stacktrace is hard to debug since it doesn't contain any context (endpoint, invalid data, etc.)

Instead we should throw an exception that contains more context and can be properly handled in the exception handler and responded to.

1. Invalid request body throws a specific exception
2. The default exception handler, responds with 400

Start at the request parsing path used by the .body method or body injection, then trace how ParseException and MapperWrongTypeException reach the default exception handler. Add context to the request-body failure and verify that invalid bodies produce a specific exception and a 400 response.

Written by the indexing model from the issue text.