lua: fix assertion failure after an error in box.session.su()

If some error occured during execution of a function called from
box.session.su(), we assumed that fiber diagnostics area was not empty,
and tried to print an error message using data from the diagnostics.
However, this assumption is not true when some lua error happens.
Imagine such a case:

  box.session.su('admin', function(x) return #x end, 3)

A lua error would be pushed on the stack but the diagnostics would be
empty, and we would get an assertion failure when trying to print the
error message. Handle this by using lua_error() instead of luaT_error().

Closes #3659