Handle all paths in OpenAPI request validation

Overview

In ApiMessageValidatorBase:

    catch (NoPath $e) {
      // @todo Temporarily log and ignore missing paths. Once 'openapi.yml' is
      //   is complete, remove this to treat them as failures.
      $this->logger->debug($e->getMessage());
    }

This has already caused us to introduce some bugs into HEAD: [##3494388]

Let's remove it and validate all requests.

Proposed resolution

Remove the catch clause, fix any issues that it uncovers.

User interface changes