Improve Error Message for Invalid Source Path

[Shion1305]

Is your feature request related to a problem? Please describe.

If I specify an invalid file path for migrate.New as the source path, the error message will look like this.

.: The system cannot find the path specified.

The error message does not contain a source path, so it is hard to determine the error from this message.
Path . comes from a hard-coded part as follows, so the error does not provide a valuable message.

migrate/source/file/file.go

Lines 31 to 34 in 04b36eb

	if err := nf.Init(os.DirFS(p), "."); err != nil {
	return nil, err
	}
	return nf, nil

It would be better if the error message contained a specified source path or if the error message said the source path was wrong.