[core/bash_impl] Use error_code_t to return error

[akinomyoga]

#2167 (comment)

Also I think it would be nice to have

if error == bash_impl.IndexOutOfRange

instead of hard-coding 1 . Then we don't need the comment, and it is more readable

This introduces an enum type to return the error status of a negative out-of-bound index of indexed arrays. To add the enum, I added error_code = OK | IndexOutOfRange in runtime.asdl (which may be extended to include other types of errors in the future). If there is a better way, please let me know.

[andychu]

This looks great, thank you! ASDL is nice to use like this

Given what I wrote in:

#2168

I think eventually we will have

OK | BadNegativeIndex | BadPositiveIndex

because bash happens to behave differently in these cases.

But I guess we can do that when we get to it