Running without `--schema-dir` probably should throw error if default dbschema directory was not found

Right now I have my dbschema in subdirectory, and, if I eventually run some migration-related command, like ` edgedb migration apply --dev-mode` it ends up with error like `No schema changes detected.`, or it just don't apply anything. It's confusing and probably should throw an error if CLI didn't find any sources.

Start by reproducing `edgedb migration apply --dev-mode` from a project whose schema is in a subdirectory and no `--schema-dir` is supplied. Trace how the CLI selects the default dbschema directory and handles missing sources; done means it reports a clear error instead of silently proceeding or saying no schema changes were detected.

Written by the indexing model from the issue text.