trying to push with the wrong macaroon return non user-friendly error

[Leo Arias]

I used snapcraft enable-ci travis to get a macaroon for the bitcoin snap. I didn't know this macaroon was attenuated only to work with this snap, so I tried to use it to push also bitcoin-unlimited. It failed with a 403 error and the json returned from the store:

Pushing 'bitcoin-unlimited/bitcoin-unlimited_master_amd64.snap' to the store.

Received 403: '{"status": 403, "error_list": [{"message": "Permission is required: package_upload", "code": "macaroon-permission-required"}], "snap_id": "qXB7ybG8qAt0zbILzctSuWOKDUfjje8i", "title": "Macaroon missing required permission.", "permission": "package_upload", "type": "devportal:v1:macaroon-permission-required", "detail": "Permission is required: package_upload"}'

It should return a user-friendly error message instead.