Wrong error returned by `luatest.Server.exec` if transaction was active

[locker]

If an error is thrown by the executed function while transaction is active, luatest.Server.exec returns a wrong error: "Transaction is active at return from function".

To reproduce the issue, add the following test to the Tarantool repository and run it:

local server = require('test.luatest_helpers.server')
local t = require('luatest')
local g = t.group()

g.before_all = function(cg)
    cg.server = server:new({alias = 'master'})
    cg.server:start()
end

g.after_all = function(cg)
    cg.server:stop()
end

g.test_error_in_tx_net_box = function(cg)
    local c = require('net.box').connect(cg.server.net_box_uri)
    t.assert_error_msg_contains(
        "My error", c.eval, c,
        [[box.begin() error("My error")]])
end

g.test_error_in_tx_eval = function(cg)
    t.assert_error_msg_contains(
        "My error", cg.server.eval, cg.server,
        [[box.begin() error("My error")]])
end

g.test_error_in_tx_exec = function(cg)
    t.assert_error_msg_contains(
        "My error", cg.server.exec, cg.server,
        function() box.begin() error("My error") end)
end

You'll get:

$ luatest -v test/box-luatest/error_in_tx_test.lua
Running with --shuffle group:7246
Started on Fri Aug 26 11:10:58 2022
    box-luatest.error_in_tx.test_error_in_tx_exec ... (0.001s) fail
...arantool/tarantool/test/box-luatest/error_in_tx_test.lua:28: Error message does not contain: "My error"
Error message received: "Transaction is active at return from function"

    box-luatest.error_in_tx.test_error_in_tx_net_box ... (0.005s) Ok
    box-luatest.error_in_tx.test_error_in_tx_eval ... (0.001s) Ok
=========================================================

Failed tests:
-------------

1) box-luatest.error_in_tx.test_error_in_tx_exec
...arantool/tarantool/test/box-luatest/error_in_tx_test.lua:28: Error message does not contain: "My error"
Error message received: "Transaction is active at return from function"

stack traceback:
        ...arantool/tarantool/test/box-luatest/error_in_tx_test.lua:28: in function 'box-luatest.error_in_tx.test_error_in_tx_exec'
        ...
        [C]: in function 'xpcall'

Ran 3 tests in 0.232 seconds, 2 succeeded, 1 failed

=========================================================
Failed tests:

box-luatest.error_in_tx.test_error_in_tx_exec

Note, neither net.box.eval nor luatest.Server.eval is affected (probably fixed in tarantool/tarantool#7288).