Log original exit code used when a builtin returns a negative exit code

[tyilo]

Description

In fish 3.7.0 executing exit with a negative argument results in:

$ fish -c 'exit -123'
warning: builtin exit returned invalid exit code 133

It would make more sense if the error was:

$ fish -c 'exit -123'
warning: builtin exit returned invalid exit code -123

TODOs:

• Changes to fish usage are reflected in user documentation/manpages.
• Tests have been added for regressions fixed
• User-visible changes noted in CHANGELOG.rst

[faho]

Makes sense, thanks!