[Improvements]Error handling completed for builtins.go file

[pratikmota]

Changes:

1. All remaining Error handlings completed for builtins.go
2. removed unused return nil
3. added TODODOC at some place
4. Added TODO-FIXME at some place
   Rye-itself//needs : Inside if condition - logic need to write. Added TODO-FIXME
   ncols : for Dic and Block need to add logic so added TODO-FIXME
5. Observation
   ioutil.ReadFile - It is deprecated in go 1.16
   Now os.ReadFile function is introduced for the latest go version.
   In future, we need to replace given functions.

[refaktor]

This is great! Core builtins are done ... I will be adding the docstrings and other fixes that you marked for me. Thank you.