feat: look for trys on falliable functions with empty error unions

[DonIsaac]

This pathological code compiles when it shouldn't.

const std = @import("std");

fn foo() !u32 {
 return 1;
}

fn bar() void {
  const i = try foo();
  std.debug.print("foo() returns {d}\n", .{i});
}

We should add a rule checking falliable functions and make sure they do one of the following:

1. Return an error
2. Use try
3. Return a call to a function that returns an error union

Item 3 is not fully doable without a type checker, but we can allow cases that return any function call.