Fix SET object/block block assuming /any (#1763)

SET without /any wasn't triggering the error it should have when
assigning a block of values that include unsets to a block or an
object. This testsfor that condition and triggers the error before
even a single assignment happens, all or nothing.

See http://issue.cc/r3/1763 for the ticket.

This also fixes http://issue.cc/r3/1515 for
    SET object block-with-unwinds
but we need a more comprehesive fix for this issue in Set_Value and
Do_Next.