Provide a helpful error message if `rockcraft.yaml` contains a `platforms.<platform>.build-on` which is not a list. Fixes #632. - [X] Have you signed the [CLA](http://www.ubuntu.com/legal/contributors/)? * \* my employing organization should have signed it AFAIK. Example of change ----- Given this snipped of a `rockcraft.yaml`: ```yaml ... platforms: arm64: build-on: amd64 ... ``` ### Before: ```bash $ rockcraft build --debug Bad rockcraft.yaml content: - error for platform entry 'arm64': value is not a valid list (in field 'platforms') For more information, check out: https://documentation.ubuntu.com/rockcraft/en/stable/reference/rockcraft.yaml ``` #### After: ```bash $ rockcraft build --debug Bad rockcraft.yaml content: - error for platform entry 'arm64': 'build-on' field must be a list of strings. (in field 'platforms') For more information, check out: https://documentation.ubuntu.com/rockcraft/en/stable/reference/rockcraft.yaml ```