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

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