Different error message if I include an exception in Request rules

I have the following update function for my model:

public function update(PartnerRequest $request, $id)
{
    $partner = Partner::findOrFail($id);

    if(!$partner -> update($request->all())) {
        throw new HttpException(500);
    }

    return response()->json([
            'status' => 'ok'
        ], 201);

}

And here is the request file:

namespace App\Api\V1\Requests;

use Config;
use Dingo\Api\Http\FormRequest;

class PartnerRequest extends FormRequest
{
    public function rules()
    {
        return Config::get('boilerplate.partner_store.validation_rules');
    }

    public function authorize()
    {
        return true;
    }
}

And here my boilerplate file:

'partner_store' => [
    'validation_rules' => [
        'name' => 'required|unique:partners,' . $this->id . ',id',
        'email' => 'email'    
    ]
]

If I remove the exception on the rule and make it as 'name' => 'required|unique:partners', I have the following error message in case of duplicate inputs:

{
  "error": {
    "message": "422 Unprocessable Entity",
    "errors": {
      "name": [
        "The name has already been taken."
      ]
    },
    "status_code": 422
  }
}

But the problem is on Patch, where I cannot update the model (due to the unique name rule). So, I need the exception. However, if I include it, instead of the above specific error, I have a generic one like this:

  "error": {
    "message": "SQLSTATE[23000]: Integrity constraint violation: 19 UNIQUE constraint failed: partners.name (SQL: update \"partners\" set \"name\" = test, \"updated_at\" = 2017-03-03 10:47:53 where \"id\" = 3)",
    "code": "23000",
    "status_code": 500
  }
}

via Tasos