box: improve built-in module load panic message

Box built-in modules, such as session, tuple, schema, etc, were
loaded using luaL_loadbuffer() + lua_call(). Error of the former
call was handled properly with a panic message describing the
problem.

But if lua_call() failed, it resulted into 'unknown exception' in
main.cc. Not very helpful. Now it is lua_pcall(), and the error
message is included into the panic() message. That helps in debug,
when something is being changed in the box modules.