Add a shortcut annotation for adding a `nil,string` return type to functions (`@error`)

Nobody has claimed this yet.

Returning of errors from Lua functions is often done by returning nil + error message (`nil,string`):

```lua
---@return table
---@overload fun(): nil, string
local function doSomething()
```

but this syntax can get complex when function takes parameters, or has other overloads. An old solution from [LDoc](https://lunarmodules.github.io/ldoc/manual/manual.md.html) was to use a `@error` annotation - I propose to add something similar to LuaLS.

The payload names no LuaLS files, tests, or entry points; start by reading the linked LDoc manual's `@error` behavior and locating LuaLS's annotation parsing and return-type handling. Done should define and test how `@error` represents a `nil,string` return for parameterized functions and functions with other overloads.

Written by the indexing model from the issue text.