box/error: don't allow overflow of error ref counter

There's no overflow check while incrementing error's reference counter
in error_ref(). Meanwhile, stubborn users still may achieve overflow:
each call of box.error.last() increments reference counter of error
residing in diagnostic area. As a result, 2^32 calls of box.error.last()
in a row will lead to counter overflow ergo - to unpredictable results.
Let's fix it and introduce dummy check in error_ref().