Fix duplicated error output

[dienhoa]

Currently, because the traceback is built from stderr, in the error case, if we only check for the existence of result.stderr like the current version does, we will print the error twice.

In [2]: print(bad)
[User] error: Expression [1]:1:7: use of unknown declaration 'bad'
print(bad)
      ^~~

(null)
[User] error: Expression [1]:1:7: use of unknown declaration 'bad'
print(bad)
      ^~~
(null)

Note: This new approach with IOHandler doesn’t have a fine-grained execution status. So we could get an error in the future if mojo repl outputs warnings to stderr (it doesn’t seem to be the case right now). I’ve asked about it on the Mojo forum, and will update if they respond with something new: https://forum.modular.com/t/mojo-repl-warnings-and-execution-status/3024

@jph00