Another from the "why are we asserting instead of doing something
sensible" department.
The alternative is to make exit() and return() compute their own exit
code, but tbh I don't want any *other* builtin to hit this either?
Fixes#9659
This skipped printing a "^" line if the start or length of the error
was longer than the source.
That seems like the correc thing at first glance, however it means
that the caret line isn't skipped *if the file goes on*.
So, for example
```fish
echo "$abc["
```
by itself, in a file or via `fish -c`, would not print an error, but
```fish
echo "$abc["
true
```
would. That's not a great way to print errors.
So instead we just.. imagine the start was at most at the end.
The underlying issue why `echo "$abc["` causes this is that `wcstol`
didn't move the end pointer for the index value (because there is no
number there). I'd fix this, but apparently some of
our recursive variable calls absolutely rely on this position value.
Prior to this fix, fish was rather inconsistent in when $status gets set
in response to an error. For example, a failed expansion like "$foo["
would not modify $status.
This makes the following inter-related changes:
1. String expansion now directly returns the value to set for $status on
error. The value is always used.
2. parser_t::eval() now directly returns the proc_status_t, which cleans
up a lot of call sites.
3. We expose a new function exec_subshell_for_expand() which ignores
$status but returns errors specifically related to subshell expansion.
4. We reify the notion of "expansion breaking" errors. These include
command-not-found, expand syntax errors, and others.
The upshot is we are more consistent about always setting $status on
errors.
