fix creating concurrent migration file

[0legovich]

Closes #707

[obalunenko]

I suggest replacing this with the code mentioned in the documentation to os.IsNotExist function
!errors.Is(err, fs.ErrNotExist)

[obalunenko]

Also this code could be simplidfied in fort of fast failure:

_, err := os.Stat(path)
if err == nil {
	return fmt.Errorf("failed to create migration file: %w", os.ErrExist)
}

if !errors.Is(err, fs.ErrNotExist){
        return fmt.Errorf("failed to create migration file: %w", err)
}