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

[0x241F31]

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.