fix: clearer error when the migrations source has no files

[AlisinaDevelo]

Closes #1314.

An empty or mistyped migrations directory currently fails with first .: file does not exist, which gives no hint about the real cause. This returns a new ErrNoMigrationFiles instead. It still wraps os.ErrNotExist, so existing errors.Is checks keep working.

The same cryptic error came up via Up() and Migrate()/Steps() since they share the code path, so both are covered. Added a test.

[coveralls]

coverage: 54.442% (+0.01%) from 54.432% — AlisinaDevelo:fix-empty-source-error into golang-migrate:master

[AlisinaDevelo]

Checking in on the no-migration-files error. It returns a typed error while preserving os.ErrNotExist wrapping, and the Go 1.24 and 1.25 tests and lint checks are green. If there’s a compatibility concern or a preferred error shape, I’m happy to adjust; otherwise I’d appreciate a review.

[AlisinaDevelo]

Hey, just checking in on this one—does the error-shape change need anything else from me before review?

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