Document `level` argument of `box.error` and `box.error.new`

Related dev. issue(s): https://github.com/tarantool/tarantool/issues/9792

Product: Tarantool
Since: 3.1
Root document:

• https://www.tarantool.io/en/doc/latest/reference/reference_lua/box_error/error/
• https://www.tarantool.io/en/doc/latest/reference/reference_lua/box_error/new/

SME: @ locker

Details

Now, when used with a table argument, box.error and box.error.new
also accept an optional second argument called level. It has the same
meaning as the level argument of the built-in Lua function error:
it specifies how to get the error location. With level 1 (the default),
the error location is where box.error / box.error.new was called.
Level 2 points the error to where the function that called box.error /
box.error.new was called; and so on. Passing level 0 avoids addition
of location information to the error.

Example of using level with box.error:

local json = require('json')

local function inner(level)
    box.error({message = 'test'}, level)    -- line:4
end

local function outer(level)
    inner(level)                            -- line:8
end

local ok, err
ok, err = pcall(outer)
print(json.encode(err.trace))               -- prints line:4
ok, err = pcall(outer, 1)
print(json.encode(err.trace))               -- prints line:4
ok, err = pcall(outer, 2)
print(json.encode(err.trace))               -- prints line:8
ok, err = pcall(outer, 0)
print(json.encode(err.trace))               -- prints empty table

Example of using level with box.error.new:

local json = require('json')

local function inner(level)
    local err = box.error.new({message = 'test'}, level)    -- line:4
    return err
end

local function outer(level)
    local err = inner(level)                                -- line:9
    return err
end

local err
err = outer()
print(json.encode(err.trace))               -- prints line:4
err = outer(1)
print(json.encode(err.trace))               -- prints line:4
err = outer(2)
print(json.encode(err.trace))               -- prints line:9
ok, err = pcall(outer, 0)
print(json.encode(err.trace))               -- prints empty table

It is also possible to specify level when using box.error to
re-raise an error created earlier with box.error.new, for example:

local json = require('json')

local err0 = box.error.new{message = 'test'}    -- line:3

local function raise(err, level)
    box.error(err, level)                       -- line:6
end

ok, err = pcall(raise, err0)
print(json.encode(err.trace))               -- prints line:3

ok, err = pcall(raise, err0, 1)
print(json.encode(err.trace))               -- prints line:6

ok, err = pcall(raise, err0, 0)
print(json.encode(err.trace))               -- prints empty table

Requested by @ locker in https://github.com/tarantool/tarantool/commit/25cdabe40dc09485d9184a80d1eb972648339ed8.

Start with the two root documents for box.error and box.error.new listed in the issue. Document the optional level argument, its defaults and location behavior, and incorporate the supplied examples as appropriate. Done means both references explain level 0, level 1, higher levels, and re-raising an existing error.