fix: capture:wrap boolean and table with errorable __index

[d-enk]

Output correct message instead crash for:

error(false)

--- false
...
stack traceback: ...

and

error(setmetatable({}, { __index = error })) -- object with field validation

--- []
...
stack traceback: ...

Current behavior

/luarocks/share/tarantool/luatest/runner.lua:259: error in error handling
stack traceback:
        /luarocks/share/tarantool/luatest/capturing.lua:81: in function </luarocks/share/tarantool/luatest/capturing.lua:74>
        [C]: in function 'bootstrap'
        /luarocks/share/tarantool/luatest/runner.lua:259: in function </luarocks/share/tarantool/luatest/runner.lua:258>
        [C]: in function 'xpcall'
        /luarocks/share/tarantool/luatest/capturing.lua:74: in function </luarocks/share/tarantool/luatest/capturing.lua:72>
        [C]: in function 'xpcall'
        /luarocks/share/tarantool/luatest/runner.lua:51: in function 'fn'
        /luarocks/share/tarantool/luatest/sandboxed_runner.lua:14: in function 'run'
        /luarocks/share/tarantool/luatest/cli_entrypoint.lua:4: in function </luarocks/share/tarantool/luatest/cli_entrypoint.lua:3>
        /luarocks/share/tarantool/rocks/luatest/0.5.7-1/bin/luatest:3: in main chunk