fix: return proper error when the grant request cannot be parsed

[hperl]

For some requests to /admin/trust/grants/jwt-bearer/issuers, Hydra would issue generic 500 error messages if decoding the request failed. This left users with no way to fix the request. Errors during decoding are now wrapped in a proper 400 Bad Request status and describe the error.

Fixes ory/network#310

Related issue(s)

Checklist

• I have read the contributing guidelines.
• I have referenced an issue containing the design document if my change
  introduces a new feature.
• I am following the
  contributing code guidelines.
• I have read the security policy.
• I confirm that this pull request does not address a security
  vulnerability. If this pull request addresses a security vulnerability, I
  confirm that I got the approval (please contact
  security@ory.sh) from the maintainers to push
  the changes.
• I have added tests that prove my fix is effective or that my feature
  works.
• I have added or changed the documentation.

Further Comments