Emit DAPException when too many variable children are reqeusted

Emit DAPException when too many variable children are reqeusted

PR dap/33228 points out a failure that occurs when the DAP client
requests more children of a variable than actually exist.  Currently,
gdb throws a somewhat confusing exception.  This patch changes this
code to throw a DAPException instead, resulting in a more ordinary and
readable failure.

The spec seems to be silent on what to do in this case.  I chose an
exception on the theory that it's easier to be strict now and lift the
restriction later (if needed) than vice versa.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=33228