Throw warning during onboarding for missing Silver tables and/or missing ddl file for source_schema_path.

[WilliamMize]

Could we add a warning to the onboarding process that would throw a warning when a table in the onboarding file is used but the corresponding silver transformation table couldn't be found? It may also make sense for the source_schema_path to be listed, but either the file isn't found or is blank. That might add some QoL when prototyping and testing new configurations.