Unify stacture downstream error handling

Currently there are 2 ways that terravis errors are handled in stacture:

Here a code property of exceptions is used: https://gitlab.eox.at/vs/stacture/-/blob/main/stacture/stacture/ows/wms.py?ref_type=heads#L430

Here the json body of the terravis response is used: https://gitlab.eox.at/vs/stacture/-/blob/main/stacture/stacture/interactions/utils.py?ref_type=heads#L550