Improve math errors

[faho]

Description

This introduces a specific error for division/modulo by zero and augments some error reporting with the length of the location.

Examples:

> math '1 - 1      pi'
math: Error: Missing operator
'1 - 1      pi'
      ^~~~~^
> math '2 + notafunction 5'
math: Error: Unknown function
'2 + notafunction 5'
     ^~~~~~~~~~~^
> math '1 - 1 / (5 - 5)'
math: Error: Division by zero
'1 - 1 / (5 - 5)'
       ^

Note: The division-by-zero error technically changes behavior, in that it now errors out early instead of carrying on and only complaining at the end that "Result is infinite" or "not a number".

That means something like

math min 1 / 0, 5

will now error out instead of returning 5.

I believe that is the correct choice - division by zero is typically expected to not be defined.

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]

In the context of a scripting language, used by an ordinary person, it's true.

When they divide by zero, it's very very likely a mistake, and so the error is helpful.

If you want to "implement a min algorithm", use the "min" function. If you want to do more complicated math, use another tool. When you're doing physics, use R or something. The right tool for the job, not your shell.

It is entirely okay if fish doesn't solve all problems everywhere. Scientific calculation is a very complicated field that we're never gonna be competitive in, and this comes at the expense of detecting legitimate errors.

Sorry, I don't want to change this.